applying a two-stage system to prioritize riparian restoration at the san luis rey river, san diego...

Applying a Two-Stage System to Prioritize Riparian Restoration at the San Luis Rey River, San Diego County, California Craig Richard Harris’

Abstract

A two-stage system for selecting stream reaches and riparian communities for restoration was applied to the 80-km San Luis Rey River below the Lake Hen- shaw dam in southern California. In the first stage, data from topographic quadrangles and aerial photographs were analyzed to define and classify reaches. These analyses concluded that (1) 28 km of the river and adjacent floodplain were suitable for second- stage evaluation of restoration needs and (2) 32 km met criteria for reference conditions at the stream reach scale and should be protected from further impacts. The remaining 20 km of the river and floodplain were considered unsuitable for restoration to reach-scale reference conditions; individual sites may be restored under existing regulatory review. Second- stage field sampling provided data on vegetation and floodplain landforms and substrate from more than 3000 plots within the 28 km of river and 1120 ha of floodplain selected for further study. Classification of floristic samples stratified by landfondsubstrate class indicated six primary riparian communities on the floodplain, some associated with particular floodplain landformhbstrate classes and others ubiqui- tous. Reference conditions for these communities

Department of Environmental Science, Policy, and Manage- ment, University of California, Berkeley, CA 94720, U.S.A. Turrent address: VESTRA Resources, Inc., Redding, CA 96002, U.S.A.

0 1997 Society for Ecological Restovation

were interpreted from the data. There were two major departures from reference conditions: tree-dominated communities were less extensive than historic levels and exotic plants had significantly invaded some landforms and communities, displacing natural communities. General goals would include restoration of tree communities and removal of exotics, with further consideration of site-specific objectives. The results included estimates of the areas by community type re- quiring restoration. The approach was developed for streams in the semi-arid western United States, but it may be adapted for use elsewhere.

Introduction

his article describes the use of a two-stage system T for selecting stream reaches and plant communities for riparian restoration. In brief, the first stage defines and classifies reaches as to suitability for protection, for management (including restoration) within existing regulatory procedures, or for restoration. The second stage uses field sampling of reaches identified for restoration to determine riparian plant communities present, associations between communities and floodplain landforms, and reference community conditions. Commu- nity structure and species composition are compared to reference conditions to identify restoration possibilities. Harris and Olson (1997) explain the system in detail. We focus here on our application of the system to the San Luis Rey River.

This two-stage system enabled us to set priorities for restoration of stream reaches and plant communities/ sites. The system addresses only riparian plant community restoration, not restoration of hydrologic or geomorphic processes. In the context of a watershed analysis, process restoration would be considered in parallel with more detailed studies for implementing riparian restoration projects. The system is meant to be general and, with adaptation, it could be applied to other rivers. Application of this system must be accompanied by a formulation of specific criteria for determining reference conditions at reach and community scales appropriate to the region and river under study.

Study Area

The San Luis Rey River watershed is a basin of 1500 km2 in northern San Diego County (Fig. 1). The study area was defined as more than 80 km of the mainstem and associated floodplain from the dam at Lake Hen- shaw (built in the 1920s) to the mouth at the Pacific Ocean. Located about 20 km below the dam, the Escon- dido Canal is designed to divert nearly all base flow

DECEMBER 1997 Restovation EcoZogy Vol. 5 No. 4.5, pp. 43-55 ~

43

Prioritizing Riparian Restoration at the San Luis Rey River

U Project Level Restoration I Reach Level Protection or Restoration

T

OCeM \ Figure 1. Location of San Luis Rey River watershed and study area, showing recommendations for project-level restoration and reach-level restoration and protection in the floodplain of the mainstem. Sections shown in the longitudinal profile were determined by manual measurements of channel gradient from Lake Henshaw to the ocean, using quadrangles (see text).

from the river, but its future operation is uncertain because of pending water rights claims on Indian reserva- tions in the watershed.

Historic maps and aerial photographs indicate that as recently as the 1930s, parts of the San Luis Rey River floodplain supported extensive riparian tree and shrub communities (Kondolf & Larson 1995). Agriculture, urban development, and other human uses have reduced the extent of riparian vegetation, particularly in the downstream third of the river. There have also been changes in the composition and structure of residual riparian communities due to land uses, invasion of exotics, and, possibly, changes in hydrology (Hawkins et al. 1997). Despite the dam at Lake Henshaw, the San Luis Rey River is subject to large floods that periodically re- arrange the active floodplain and vegetation. Such events have occurred several times over the past 50 years, most recently in the winter of 1992-93 (Kondolf & Larson 1995). Flooding may be exacerbated because parts of the floodplain and watershed have been urbanized (Hawkins et al. 1997).

The San Luis Rey River has been the locus for conten- tious land use disputes in the past few years. Some of these have centered on the presence of the endangered least Bell’s vireo, a bird that requires willow thicket habitat for nesting (Hendricks & Rieger 1989). More recently, conservation planning for coastal sage scrub habitat, multiple species planning by San Diego County, and a watershed management planning study spon- sored by the U.S. Environmental Protection Agency (EPA) have all affected the study area. We chose the

study area, in part, because of ongoing planning and regulatory activities.

Methods

This section describes the procedures involved in each of the two stages of our approach. The objective of the first-stage reach classification was to stratify the mainstem into reaches and assign the reaches to categories, based upon present condition relative to reference conditions. In the second-stage, field data were collected to evaluate restoration possiblities at plant community and site scales within a reach.

First Stage: Reach Classification

The analysis conducted to support reach classification required delineation of reaches using geomorphic data, mapping of land use cover within reaches, formulation of reference condition criteria, comparison of reach data to reference criteria, and finally, reach classification. The primary data sources were U.S. Geological Survey (USGS) 7.5-minute topographic quadrangles and aerial photographs. Auxiliary data, including field reconnaissance, were used to verify geomorphic interpretation and land cover mapping.

Reach Delineation. The channel gradient from Lake Hen- shaw to the ocean was measured manually from quadrangles and the measurements were used to prepare a stream longitudinal profile. The profile was interpreted according to methods presented in Richards (1982) to distinguish stream sections where the dominant processes were sediment transport or deposition. Within these sections, local channel slope and measurements of the geomorphically defined floodplain (including the channel, fluvial landforms, and terraces) were obtained from quadrangles and used to delineate preliminary down-valley and cross-valley reach boundaries. These boundaries were refined using auxiliary data on geol- ogy, tributary influence, and soils, along with field and aerial photograph reconnaissance (Kondolf & Larson 1995). Primary reach delineation criteria (channel slope and floodplain width) were chosen based on the as- sumption that the forces of flooding act as major con- trols on the distribution, abundance, and composition of riparian communities in the region. Valley confine- ment (i.e., floodplain width) and channel slope are good indices for the susceptibility of a stream reach to erosion, sedimentation, and scouring during peak flows (Richards 1982). Streamflow and groundwater data were reviewed; despite diversions, the San Luis Rey River is perennial in most years. Presence of groundwater is positively associated with floodplain width, with the largest groundwater basins located in the rela-

44 Restoration Ecology DECEMBER 1997

Prioritizing Riparian Restoration a t the San Luis Rey River

tively wide middle and downstream sections of the river (San Diego Association of Governments 1990). The effects of diversions and groundwater depletion or aug- mentation due to pumping and/or urban and irrigation return flows were not evaluated. As noted by Kondolf and Larson (1995), these matters would require, at minimum, study in the design and implementation of restoration projects. Results of geomorphic interpretation consisted of mapped reaches with upstream to downstream and cross-valley floodplain boundaries on both sides of the river.

Land Cover Mapping. Reach boundaries were transferred to black-and-white aerial photograph transparencies (date 1989; scale 1:31,680) and photo interpretation was used to map land cover classes occurring on the floodplain (exclusive of the low-water channel) within each reach. The 1989 photographs were the most recent available at the time this study was conducted. Land cover classes were chosen, first, to separate urban from vegetation land cover, second, to separate cultivated from riparian vegetation, and third, to separate riparian vegetation into canopy classes (tree, shrub, and herbaceous). Water (ponds) and barren land were separate land cover categories, for a total of seven classes. The minimum mapping unit was 1 ha. A polygon was assigned to a category if more than 50% of it was in the land cover. Land cover at the interface between uplands and floodplain along each side of each reach was also classified, distinguishing between urban, agriculture, and natural vegetation land covers. In this case, a map wheel was used to quantify the length of each bound- ary in each land cover. These measurements were later converted to percentages.

Formulation of Reach-Scale Reference Condition Criteria. The criteria used to evaluate riparian reference conditions must be developed specifically for the region and river. The general criteria used for this study were:

proportion of the floodplain occupied by riparian shrubs and trees; degree of fragmentation of riparian communities (number and sizes of patches per reach, adjacency of irreversible land use to riparian vegetation patches); channel slope and floodplain width; proportion of urban and agricultural development on the floodplain; and connectivity between the floodplain and upland vegetation (as measured by the proportion of the floodplain/upland border in natural vegetation land cover).

After collecting data on these variables from land cover maps, we defined quantitative and qualitative values for each. Assuming that some of the reaches were at- taining reference conditions, we selected these reaches;

their values for the criteria became the basis for comparison to other reaches. The reference criteria values were therefore specific to the San Luis Rey River. Rela- tive to existing conditions, reach reference conditions were > 90% coverage by riparian communities, occurring within a few relatively large patches with good connectivity between the floodplain and uplands (> 60% of the border in natural vegetation).

Comparison of Reach Data to Reference Criteria. Urban or agricultural land cover exceeding 30% was considered a sig- nificant departure from the reference condition, creat- ing a condition potentially unsuitable for reach-scale restoration. Reaches with 60-90% riparian cover, limited fragmentation, < 10% cover of irreversible land uses, and good connectivity to uplands were considered candidates for restoration, depending on field verification.

Reach Classification. Systematic application of these criteria enabled us to classify reaches into the following three categories:

(1) Protection or preservation. These reaches were achieving reference conditions. They had > 90% riparian vegetation cover in a few relatively large patches and good connectivity to uplands.

(2) Restoration to reach-scale reference conditions not feasi- ble. Reaches in this category would receive no further analysis. Fragmentation and incursion of irreversible land uses precludes reach-scale restoration. Individual sites may be restored in the course of normal regulatory review.

(3) Further analysis needed to establish restoration needs at the community or site scale.

Completion of the reach classification concluded the first stage of inventory and analysis. Reaches in the third category were designated for second-stage field study.

Second Stage: Evaluation of Restoration Needs

Field Study Design. The field sampling was designed to efficiently collect the data necessary for evaluating restoration possibilities at the plant community and site scales. We assumed that in western semi-arid regions, floodplain sites of similar landform and substrate are capable of supporting similar plant communities (Har- ris 1987; Szaro 1990). We further assumed that at least some’sites in the San Luis Rey would have communities and samples indicating reference conditions. The field study was designed to sample about 30 occurrences of any landform/substrate combination occupying > 1% of the study reaches. In preparation for the field work, a sampling grid of transects spanning the floodplain,

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DECEMBER 1997 Restoration Ecology 45


ending at the low-water channel and aligned according to azimuths perpendicular to the river, was marked on aerial photographs of each reach. The transect interval (152 m) and plot interval (every 30 m along transects) were chosen as a compromise between the needs for achieving complete coverage and for minimizing travel time between plots. The sampling design further re- flected the size of the study area and our observations on the sizes of landforms and riparian community patches. The first plot at each transect was selected at a random location between 0 and 30 m from the mapped edge of the floodplain. At an approximate spacing of 152 m by 30 m, more than 3000 plots were located along the 28.1 km in the eight reaches, two of which were con- tiguous, selected for field study (see Table 1 for lengths and areas of study reaches).

Field Data Collection. During the late spring and summer of 1993, data were collected in circular plots 3.5 m in ra- dius (0.004 ha) for three vegetation layers, generally corresponding to the tree (>5 m), shrub (1-5 m), and herb (<1 m) layers. Total cover was estimated as the sum of the cover in each canopy layer. Since the aim was to characterize communities rather than to identify rare species, the two dominant species were identified and recorded within each canopy class. Dominant species were defined as those having the greatest relative cover within a particular vegetation layer. Relative cover is defined as the proportion of cover occupied by a particular species relative to total cover in a canopy layer (Mueller-Dombois & Ellenberg 1974). The total number of plant species was recorded for the tree and shrub layers, but not for the herb layer because some annual species would not be present. Percent cover for each dominant species and total cover for each layer were visually estimated according to methods described by Platts et al. (1987), providing estimates within 10% of true cover values.

We also collected data on landform and surficial substrate (an index to the frequency, intensity, and dura- tion of flooding) at each plot. Landform classes followed Harris (1987) and consisted of active floodplain, depositional floodplain, erosional floodplain, and banks. Substrate was recorded as the predominant sediment size (bedrock, boulder, cobble, gravel, sand, or silt) cov- ering the plot. Conditions of interest, such as unusual geomorphic or hydrologic features or evidence of human impact, were recorded for each plot. Plots that fell in urban, agricultural, or open water (pond) areas were not sampled but were classified visually in the field according to land cover. These data were used with vege- tated riparian plots to calculate land cover proportions for each reach.

Private land predominates at San Luis Rey and access to some sample locations was denied. Rather than elim-

inating these plots, we made visual estimates of the land cover, the riparian tree, shrub and herb species present in riparian cover types, and the landform and substrate for inaccessible plots. These data were recorded on the usual field forms, with the notation that they were estimates, not measurements.

Analysis of Field Data. Three types of data were collected: (1) measured plot data, (2) classified plot data (agriculture, urban, water), and (3) estimated plot data for inaccessible plots in riparian land cover. To evaluate vegetation conditions and relationships to environmental conditions, we used Ward’s Minimum Variance cluster- ing routine (SAS Institute, Inc. 1988) to define groups of samples with similar dominant and associated species based on relative cover. This algorithm groups samples by minimizing variance within groups (or clusters) and maximizing variance between groups. The data used for the analysis consisted of relative cover values for dominant species by canopy position.

Samples were stratified into landform/substrate classes as: active floodplain, any substrate; bank, any substrate; erosional floodplain, any substrate; depositional floodplain, sand substrate; depositional floodplain, silt substrate; and other depositional floodplain. Separate cluster analyses were conducted for the samples within each of these landform/substrate classes.

The outputs from the analyses were clusters of like samples grouped at a defined level of variability for each landform/substrate class. Means and variances were calculated for the relative cover of each species, total tree, shrub, and herb cover, and tree and shrub species richness for each cluster. A cluster was named after the species that had the highest average relative cover. In the remainder of this article, we refer to the named cluster types as plant communities.

After completing the cluster analysis, we reviewed the estimated data on vegetation collected for inaccessible plots and assigned the plots to community types. This step increased the completeness of the data set and facilitated interpretation of the degree of degradation of riparian communities in the study area and reaches.

Determination of Restoration Needs. The final step in the analysis was the interpretation of plant community occurrence, structure, and composition in relation to reference conditions for each community and landform/ substrate class to determine restoration needs. Results of the cluster analysis guided the interpretation. We assumed that the presence of a shrub- or tree-dominated community in a particular landform/substrate class was evidence that this community could be established on that landform/substrate type through restoration. Conversely, the absence of a tree or shrub community in a landform/substrate class was evidence that it could


Table 1. Geomorphic and land cover characteristics of stream reaches, San Luis Rey River. Reach numbering is sequential from Lake Henshaw downstream to the Pacific Ocean.

Descriation

Channel Floodplain Reach Percent Percent Percent Slope Width Length Area Natural Urban/ Other (%) (m) (km) (ha) Vegetation* Cultivated*" Land Cover Recommendation

1.1 1.2

2.1

2.2

2.3

3.1

3.2

3.3

4.1

4.2

4.3

5.1

5.2 5.3

5.4 5.5

5.6 5.7 5.8 5.9 5.10

5.11

Section 1 V-shaped valley 0.6 V-shaped valley, partly bedrock channel, no or narrow floodplain 1.6 Section 2 V-shaped valley, bedrock channel 14.4 V-shaped valley, bedrock channel 2.7 tributary influenced V-shaped valley 0.4 Section 3 asymmetrical tributary influenced floodplain, asymmetrical steep V-shaped valley walls 0.3 symmetrical V-shaped valley 0.15 asymmetrical tributary influenced floodplain, asymmetrical steep V-shaped valley walls 0.5 Section 4 narrow incised valley in alluvial fan deposits 2.5 incised valley in alluvial fan deposits 1.4 incised valley in alluvial fan deposits, confined by Mesozoic basic intrusic rocks 0.9 Section 5 wide valley, tributary influenced floodplain 0.4 V-shaped valley 0.3 wide valley, tributary influenced floodplain 0.2

wide valley, tributary influenced floodplain 0.2

V-shaped valley 0.2

V-shaped valley 0.2 V-shaped valley 0.2 V-shaped valley 0.2 narrow V-shaped valley 0.2 V-shaped valley, marine and non-marine terrace

wide valley, marine rock and non-marine terrace

deposits 0.1

91 7.1 67 100 0

37 8.4 32 100 0

40 1.0 4 100 0

40 3.1 12 100 0

213 1.9 40 86 11

579 4.8 276 86 12

427 1.0 40 64 36

427 7.1 300 51 38

198 1.8 36 96 0

305 1.3 40 97 0

274 1.8 48 90

732 610

792 366

792 683 853 457 274

8.4 256 69 3.5 212 75

1.0 76 84 1.1 40 52

2.7 216 56 1.3 92 84 3.9 330 93 4.0 184 58 2.7 76 90

3

24 5

15 30

36 10 2

24 1

293 2.3 24 67 33

Protect 0

0 Protect

0 Protect

0 Protect

3 Restore

2 Restore

0 Project Review

11 Project Review

4 Protect

3 Protect

7 Protect

7 20

1 18

8 6 5

18 9

Restore Restore

Restore Project Review

Project Review Restore Protect Restore Protect

0 Project Review

deposits 0.1 549 3.5 192 55 37 8 Project Review 5.12 wide valley, marine rock

and non-marine terrace deposits, tributary confluences 0.1 1798 7.6 1140 38 58 4 Project Review

5.13 V-shaped, valley walls of marine and non-marine deposits 0.1 186 3.2 59 75 5 20 Restore

*For natural vegetation, > 90% cover = protect. Values that meet criteria are in bold type. **For urban/cultivated, > 30% = project review. Values that meet criteria are in bold type.


Prioritizina Riwarian Restoration at the San Luis Rev River

not be restored there. Rarity of a community was interpreted as a sign of degradation. The reference composition and structure for each community were inferred from data for each cluster within each community type. We assumed that the samples WI@ the purest composition of native riparian species and the most complex community structure represented reference conditions for that community. Reference conditions could include single-storied, single-species communities (e.g., willow thickets) or multistoried, multispecies woodlands.

Application of these guidelines led to conclusions that some communities, such as barren or herbaceous, were associated with environments subject to frequent, intensive flooding. Occurrence of barren or herbaceous communities in landform/substrate classes less subject to flooding, where other communities represented reference conditions, indicated a need for restoration. Variability in cover, species composition, and structure within each community were responses to the interactions between succession and disturbance. For example, a community dominated by willow might have occurred on several landform/substrates but might have been most commonly found on depositional floodplain with sand substrate. Its abundance there was an expres- sion of the suitability of that landform/substrate type for the community. The structure and composition of a willow community (e.g., presence of riparian trees in the understory, presence of exotic species) on that landform was indicative of other disturbance and/or successional processes.

Results and Disussion

Reach Classification. Twenty-four stream reaches encom- passing 3900 ha of floodplain were defined on the San Luis Rey River, based on geomorphic data interpretation (Table 1). As of 1989, the date of aerial photographs used in land cover mapping, urban uses occupied about 760 ha (20%), cultivated agriculture occupied about 390 ha (lo”/.), and water (ponds) and barren categories occupied about 260 ha (7%). The remainder, about 2500 ha (64%), was riparian vegetation occurring in 43 distinct patches ranging from 1 to 300 ha. Riparian vegetation varied in structure from herbaceous to closed canopy forest.

Nine of the 24 reaches were achieving a reference condition of > 90% riparian cover, limited fragmentation, good connectivity to uplands, and no irreversible land uses. These reaches were recommended for protection (”protect” in Table 1). Another seven reaches were removed from consideration for field studies because of relatively low riparian cover, relatively high cover of irreversible land uses, and high fragmentation (”project review” in Table 1). The remaining eight reaches were

identified for further study (“restore” in Table 1). This group contained reaches where some degradation had occurred that might be reversible or where actions could be taken to prevent further degradation.

Field Study Results. Within the eight reaches selected for field study, 3169 plots were either sampled or classified. This number included 1275 plots where vegetation, landform, and substrate data were collected (a few plots were omitted from some of the analyses because of missing data), 1117 plots that were classified as urban, agriculture, or water, and 783 plots (24% of the total) that were inaccessible but where visual estimates of land cover class (i.e., vegetation type) and landform/ substrate class were recorded. Although there was a suf- ficient number of measured plots in each reach, the proportions of measured, classified, or estimated plots were unequal by reach, reflecting the fact that permission for access from landowners was unequally available. Over- all, the number of sampled plots was considered ade- quate for classification of riparian communities.

Landform and Substrate Conditions. Analysis of measured plots indicated that active floodplain was 22%, depositional floodplain was 66%, and erosional floodplain was 7% of the study area. The remaining 5% was in banks. Geomorphically, the system was simple; complex landforms such as secondary channels were not present. The 1992-93 floods made a clear demarcation between the active floodplain subject to frequent scouring and the depositional floodplain. Sand was the dominant lag deposit on all surfaces except depositional floodplain, where sand and silt were nearly equally represented. Flooding did not appreciably change the distribution of landforms as evidenced in 1989 photographs, although it did remove some riparian vegetation, especially on severely scoured floodplains.

Current positions of landform/substrate combina- tions on the floodplain were not necessarily reflective of their future positions. Flashy stream systems such as the San Luis Rey River are notorious for changing course during major flood events. It was fortunate that major floods preceded field sampling, however, because the active floodplain and meander belt for the river were fairly well defined by the events. Neverthe- less, avulsions of the stream from its active floodplain can occur, reshaping surfaces that formerly experienced only deposition. This factor adds a stochastic element to restoration planning for streams in the western United States that can be anticipated through historical analysis but never fully predicted.

Vegetation. We identified 110 plant species. Of these, eight were typical riparian trees and tree/shrubs [Fuaxi- nus velutina (Arizona ash), Platanus racemosa (California



sycamore), Populus fiernontii (Fremont’s cottonwood), Quercus agrifolia (black oak), Salix gooddingii (Good- ding’s willow), s. hindsiana (valley willow), s. laevigata (red willow), and S. lasiolepis (arroyo willow)]; four were typical riparian shrubs/lianas [Baccharis salicifolia (mulefat), Rosa californica (California rose), Sarnbucus mexicanus (Mexican elderberry), Vif is girdiana (desert grape)], and seven were typical ripariadwetland herbs [Arfernesia douglasiana (Douglas mugwort), Cyperus em- grostis (sedge species), Juncus balticus, (rush species), Melilotus albus (white sweetclover), Typha latifolia (broad- leaved cattail), Scirpus californicus (California bulrush), and Xanthiurn sp. (burs)]. Two exotics common to riparian settings in the semi-arid and arid west were relatively abundant [Arundo donax (giant reed) and Tarnarix rarnosissirna (tamarisk species)].

The most common riparian indicators were: in the > 5-m layer, cottonwood and arroyo willow; in the 1-5- m layer, mulefat and arroyo willow; and in the < 1-m layer, mulefat. Annual Bromus spp. (brome grass) and Eriogonurn fasciculaturn (buckwheat), both of which are typical grassland species, were overwhelmingly the most common herbs.

Overall, species richness in tree and shrub layers was low, averaging 0.19 tree spp./plot and 2.69 shrub spp./ plot. Average tree canopy cover was also very low, at 4%. Average shrub canopy cover was 21% and herb cover averaged 52%.

Relationships Between Vegetation and Landform/Substrate.

On the active flood plain, 77% of the plots had no tree canopy layer whatsoever and 51% had no shrub cover. On the depositional floodplain, 78% of the plots had no tree cover and those plots with tree canopy were nearly

all on sand and silt substrate. However, over 93% of the plots in depositional floodplain had shrub cover. Over 83% of erosional floodplain plots had no tree cover but 83% of the plots did have a shrub layer.

The landform/substrate combination occurring on a plot was an index to the frequency and intensity of flooding. Vegetation cover data indicate that active floodplain was the most severe environment; depositional floodplain was a less severe environment, with silt substrate indicative of settling under still water conditions during the most recent floods. Erosional floodplain was intermediate in severity of flood effects.

Eighty-three percent of all plots were devoid of tree cover, although tree cover varied by reach. The general paucity of tree cover is hypothesized to be the result of the cumulative impacts of flooding, possible alterations of groundwater regime, grazing, and clearing for agriculture and urban development. We had no historical data to establish the relative impacts of these activities.

Plot Vegetation Classification. Cluster analyses stratified by landform/substrate classes produced six plant communities (Table 2). Some of these communities occurred across the floodplain, irrespective of landform/substrate type, whereas others indicated differences in environmental conditions. For example, the cottonwood community was especially prevalent, and a good indi- cator of depositional floodplain with sand substrate, while the herbaceous community was absent.

The distribution of plant communities by landform (Table 2; combined substrates) indicates severity of flooding and other disturbances in the different landform/substrate types, as well as potential targets for restoration (Fig. 2). Active floodplain was almost com-

Table 2. Plant community nomenclature and characteristics. Average cover and species richness values are the ranges observed in clusters across landform/substrate classes.

No. of Average Average Cover of Average Species Community Type Plots LandformlSubstrate Classes Cover (%) Dominants (%) Richness/Plot Exotics Present

Mulefat shrub 128

Willow shrub 182

Oak Savanna 238

Cottonwood forest 191

Herbaceous 119

Scoured/Barren 411

ALL 62-115

ALL 100-176

Depositional floodplain 88-110 Erosional floodplain Banks Depositional floodplain 50-91 Banks All but depositional 65-130

floodplain/sand

floodplain / sand Active floodplain Erosional floodplain Banks

Depositional 20-71

30-51 2.7-4.4 Saltcedar

10-63 3.3-6.0 Saltcedar Giant reed

Giant reed 3-27 1.2-2.7 None abundant

4-6 2.1-3.2 None abundant

- 2.8-6.5 Saltcedar

- 1-2.8 None abundant Giant reed


Prioritizing Riparian Restomtion at the San Luis Rey River

Figure 2. Proportions of classified plant communities occurring on floodplain land forms, San Luis Rey River field study area.

pletely occupied by scoured/barren or herbaceous communities (88%), with small areas of mulefat and willow shrub communities. Erosional floodplain was 63% scoured or herbaceous, about one-third mulefat or willow, with a minor amount of oak savanna. Depositional floodplain was nearly half riparian forest (oak savanna and cottonwood). Only 23% was in scoured or herbaceous communities. Composition and structure of the different communities varied by landform/substrate, reflecting successional and disturbance effects. For example, giant reed was found in several communities, but was well established in the mulefat community on active floodplain and in the willow community on all but one landform/substrate (depositional floodplain/sand). In the willow community on erosional floodplain, giant reed was present in all three canopy layers. Advanced degradation of the willow community was indicated by samples in which the cover of giant reed in the shrub and herb layers exceeded cover of willow.

Community Reference Conditions. Reference conditions for active and erosional floodplains are mulefat or willow communities with relatively high total cover (up to loo%), composed of at least 50% relative cover of mulefat and/or willow species. Exotics are absent. Reference conditions for depositional floodplain are cottonwood forest, with moderate total cover (up to 75%) and about 5% relative cover of cottonwood. Stands may be two- storied or more. Exotics are absent. Additional reference communities for depositional floodplain are willow (as described previously) or oak savanna. Reference values for the oak savanna are relatively high total cover (up to loo%), composed of about 20% relative cover of oak. Stands may be two-storied or more, with native shrubs in the understory. There were no reference stands with native grasses in the herb layer.

Land Cover Distribution. There were 2058 measured or visually estimated plots in riparian vegetation. Each visually estimated plot in riparian vegetation was assigned to the most similar vegetation class. In addition, there were 356 urban plots, 647 agriculture plots, and 114 plots with water (ponds) in the study area. Analysis of the total data set indicated the distribution of land cover for the floodplain as a whole and by study reach, shown in Tables 3 and 4, respectively.

The results obtained from field sampling were compared to the results obtained from photo interpretation conducted during the first stage. Three reaches (5.1,5.6, and 5.8) had higher proportions of urban and cultivated agriculture land cover than indicated by photo interpretation (see Tables 1 and 4). In general, differences between estimates obtained from aerial photo interpretation for purposes of reach classification and estimates obtained from field sampling appeared to be due primarily to different levels of resolution between photo coverage and field work and the different classification systems used. Field work revealed that in some cases, especially reach 5.6, photo interpreted herbaceous com-

Table 3. Percentages of floodplain and estimated areas of land cover types for reaches subject to field study, San Luis Rey River.

Land Cover/ Community Type

Percen tage of Floodplain

Estimated Area (ha)

Mulefat Willow Oak Cottonwood Herbaceous, Scoured Urban Agriculture Water Total

5 7 8 9

16 20 11 20 4

100

55 77 88 99

176 220 121 220 44

1120

50 Restoration Ecology DECEMBER $997

Prioritizing Riaarian Restoration at the San Luis Rew River

Table 4. Percentages of reaches subject to field study in land cover classes, San Luis Rey River.

Reach Mulefat Willow Oak Cottonwood Herb Scoured Urban Awic. Water

2.3 26 2 16 13 10 31 0 0 2 3.1 8 1 13 9 34 15 12 8 0 5.1 6 6 11 9 13 20 7 28 0 5.2/3 1 8 2 9 18 21 3 25 12 5.6 3 2 4 8 0 10 22 42 8 5.8 2 9 8 9 8 23 30 9 1 5.13 4 60 3 1 4 26 2 0 0

munities were actually irrigated agriculture. The minimum mapping area for land cover mapping was 1 ha, while the resolution of field sampling was 0.004 ha. As would be expected for the higher resolution field sampling, it provided more accurate estimates of distribution of the various land cover and community types within the study reaches.

Interpretation of Restoration Needs

Overall Status of the San Luis Rey Floodplain. The system pro- vides a conservation strategy for the entire floodplain that, if followed, would make protection and restoration a high priority for 60 km of stream and associated floodplain. The remaining 20 km of stream and floodplain were judged too impacted for restoration to reach- scale reference conditions. However, individual sites might be restored in that 20 km during normal project review procedures. Most of the 20 km area is in the urbanized downstream reaches and some upstream reaches currently used for instream gravel mining.

Further site-specific studies would be required to implement restoration projects in the eight reaches where they are recommended. If they were successful, these projects would improve connectivity of the riparian corridor in three areas of the basin (Fig. 1; Table l), which would probably benefit water quality and wildlife. A continuous riparian corridor extending for over 26 km downstream from Lake Henshaw would include sections 1 and 2 and part of 3. Restoration in reaches 5.1, 5.2, and 5.3, coupled with protection of upstream reaches in section 4, would create another corridor of nearly 18 km. These two corridors would be separated by 8.1 km of highly developed and disturbed floodplain in reaches 3.2 and 3.3. Project-level mitigation planning for restoration of reaches 3.2 and 3.3 could be coordi- nated to protect and enhance upstream and downstream protection and restoration efforts.

Restoration of reaches 5.6 and 5.8, complemented by protection of reaches 5.7 and 5.9, would create a third corridor of nearly 12 km. Downstream from this area are more than 13 km of developed floodplain. The last reach before the ocean, 5.13, would be restored. Mitiga-

tion for projects upstream from reach 5.13 should be planned so that they would not have negative effects on downstream restoration efforts.

It should be noted that this study did not assess the need for geomorphic or hydrologic modifications that might be necessary to support restoration of riparian communities. For example, the floodplain mapped for reach 5.13 is confined within levees. Urban uses extend across the remainder of the geomorphic floodplain. We did not include the urbanized floodplain within the study area because we considered removal of the levee to be infeasible. On other parts of the stream where levees exist but the floodplain has not been urbanized, levee removal might be an option. Of necessity, these important issues would have to be addressed during further planning and design work. Other activities, such as gravel mining, diversions, and groundwater pumping, may significantly affect existing riparian community conditions and feasibility of restoration. We did not attempt to determine the effects of these activities.

Reach-Scale Restoration. Within the eight reaches recommended for restoration, the six plant communities (Table 2) vary in composition and structure, depending on the landform/substrate on which they occur, the presence of exotics, and past or present land uses. In general, tree communities are relatively rare, and where they occur, tree cover appears to be reduced relative to historical conditions. Historical aerial photographs indicate that many locations, especially on depositional floodplain now dominated by a herbaceous community or other uses, were once dominated by trees (Kondolf & Larson 1995). It is likely that scoured/barren, mulefat, and willow communities are distributed in locations much as they always were. These communities are indicative of frequent, intense flooding (Campbell & Green 1968; Harris 1987) and are characteristic of locations where restoration projects would be subject to de- struction in the future. The active floodplain is not an area where restoration for riparian tree cover would be cost effective; however, some measures can be taken to modify the composition of shrub communities on the active floodplain (i.e., control exotics) and thus restore



reference conditions. Depositional floodplain is also subject to occasional intense flooding, but the probabil- ity of destructive flooding is low and recurrence inter- vals may exceed the economic life of a restoration project.

Results suggest two overarching goals for restoration in the eight reaches: increase the area of tree-dominated communities to restore historic patterns and reduce the impact of exotics through selective vegetative management. Tree communities that exist could also be man- aged for increased tree and shrub cover through vegetation manipulation. These goals should be subject to review in relation to restoration objectives for improv- ing ecological functions. These concepts are further developed in the sections that follow.

Using the landform proportions developed during field sampling, active floodplain, wherein active restoration is not recommended, constitutes 22% of the reaches, or 246 ha. Erosional floodplain, another risky setting for restoration, is 7%, or 78 ha. Depositional floodplain, the primary locus for active restoration, is 739 ha, representing the maximum area for restoration activities within the studied reaches. We have no data to indicate the stability of these proportions, but they are comparable to at least one other stream that we have studied (Harris 1987).

Continued restoration planning should focus on the specific ecological functions that are to be protected, en- hanced, or restored, both for the river as a whole and for specific reaches. Therefore, the community-based recommendations that follow are offered with the understanding that other priorities may ultimately prevail in the watershed.

Community Restoration and Protection. Four communities are targets for restoration: mulefat shrub, willow shrub, oak savanna, and cottonwood forest. The most suitable landform for restoration activities is depositional floodplain. Both active floodplain and erosional floodplain, where willow and mulefat are the reference communities, are subject to frequent, intense flooding that might destroy restoration projects. Within the depositional floodplain, areas presently occupied by herbaceous communities (about 140 ha) would be the principal place for concentrating the establishment of reference riparian shrub and tree communities (willow, cottonwood, and oak). There are also restoration needs within existing riparian communities, mainly controlling giant reed and tamarisk, but including vegetation management to maintain certain conditions that additional site- specific studies may determine to be desirable.

Table 5 presents a summary of recommendations for the four communities. Control of giant reed would be necessary in about 10 ha of mulefat community. We do not recommend expanding this community through ac-

tive planting; periodic scouring by floods should pro- vide for the continued regeneration of the community.

The rarity of the willow shrub community is of seri- ous concern in the watershed because of the important role the community plays as nesting habitat to the endangered least Bell's vireo (Hendricks & Rieger 1989). An aggressive program to control giant reed would be necessary in an area estimated at 70 ha. Increasing the size and connectivity of willow patches (at the expense of herbaceous communities) and vegetation management to prevent succession to tree-dominated communities (through coppicing of willow clumps and selective removal of cottonwood) are options. The latter action would have to be considered in full recognition of the tradeoffs involved.

Oak savanna is relatively abundant in the upstream study reaches (Table 4) where it is also common on sideslopes adjacent to the floodplain. Threats to regeneration of oak communities include grazing and/or competition with annual grasses (Borchert et al. 1989). The understory grasses are primarily Mediterranean grasses and forbs. A restoration objective based on re- placing these with native perennial and annual grasses could be evaluated in conjunction with expansion of the community through planting.

Cottonwood riparian forest is moderately rare in the study reaches, but its total area (99 ha) is distributed almost equally among the reaches (Table 4). It is confined mostly to depositional floodplain/sand where it is relictual from formerly more extensive communities. Low values for tree canopy cover (4-6%), total cover, and species richness are not necessarily indicators of disturbance in this community; similar values are recorded for relatively natural cottonwood-dominated communities on an alluvial stream in Califor- nia's Central Valley (Harris 1987; 9-14% tree cover; 15-37% total cover). Absence of exotics in the type is a positive sign.

Specific conditions are required for natural regeneration of cottonwood (Brady et al. 1985; McBride & Stra- han 1985), and artificial regeneration through planting is commonly used as a restoration practice. An objective to restore cottonwood forest to locations on depositional landforms now dominated by herbaceous communities would have to be weighed against other aims, including the need for restoration of willow communities. Conversion of other communities to cottonwood forest could be accelerated through vegetation management. Otherwise, existing cottonwood stands could be used as nodes for expanding the type through planting, as suggested previously for the oak savanna type.

In summary, community-scale restoration activities could focus on: (1) eliminating encroachment of exotics in the mulefat and willow communities in an area of about 80 ha, (2) expanding willow community patches


Prioritizing Riparian Restoration at the San Luis R q River

Table 5. Summarv of dant communitv restoration recommendations. San Luis Rev River.

Floodplain Restoration Goals: Increase area of riparian forest and reduce impact of exotic plants on riparian shrub and tree communities.

Community Functions Threats Recommendations

Mulefat shrub Floodplain stabilization (roots). Sediment screening (stems). Flood attenuation (stems). Nesting and foraging habitat

for least Bell’s vireo.* Same as mulefat shrub. Nesting habitat for least Bell’s

Willow shrub

vireo.**

Oak savanna Wildlife habitat

Cottonwood forest Wildlife habitat

Invasion of exotics. Control exotics on approximately 10 ha. Maintain periodic flooding to ensure

natural regeneration.

Invasion of exotics. Fragmentation (scarce except

Control exotics on approximately 70 ha. Increase size and connectivity of willow

Maintain community through vegetation management to prevent successional changes.

Maintain periodic flooding to encourage natural regeneration.

Replace annual grasses with native perennial grasses (experimental).

in reach 5.13). patches.

Impaired regeneration due to grazing and competition from annual grasses. Exclude grazing.

Increase area of existing stands through

Increase area of existing stands through planting.

planting. Clearing for agriculture?

* Olsen & Gray (1989); Hendricks & Rieger (1989); Kus &Miner (1989). ** Hendricks & Rieger (1989).

to enhance habitat for the least Bell’s vireo, and (3) expanding oak savanna and cottonwood forest on depositional floodplain in areas now occupied by herbaceous communities in an area of about 140 ha. We offer these recommendations with the understanding that the desirable mix of communities is unknown. To determine that mix, multiple resources and functions must be considered and overall objectives must be established. A case in point has previously been illustrated with the discussion of willow community restoration. Moreover, the feasibility of implementing specific projects would depend on further planning and design, including thor- ough evaluation of hydrologic processes required to sustain the restored communities.

Conclusions, Limitations, and Recommendations

The results of this case study may be useful to local and regional planners, but that was not the primary objective of the project. The primary objective was to apply a system for restoration planning at watershed scale to a specific situation and determine if the results were con- gruent with results suggested by available information. The results obtained from this study generally concur with published and unpublished accounts of the condition of riparian resources in the San Luis Rey watershed (Faber 1989; Hendricks & Rieger 1989; San Diego Asso- ciation of Governments 1990). Exotics are known to be a

problem there and in other nearby watersheds. The plant communities indicated by our classification are common in southern California and their environmental relationships are fairly well documented (Faber 1989). Results similar to those reported in this study have been obtained through historical studies (Kondolf & Larson 1995). The advantage of this system over historical analysis is that it does not depend on historical data that may or may not be available for an entire stream. Kondolf & Larson (1995) confined their study to a few reaches where such data were available.

The approach used for this research is meant to be generally applicable, especially to watersheds in the western United States where whole-river restoration planning is desirable. Experience in applying the approach to the San Luis Rey watershed has revealed some limitations and some areas for possible improve- ments.

(1) It will be difficult to obtain access to some areas in mixed-ownership basins. Our contingency sampling plan sufficed in the present study, but it might not work in other study areas. It is advisable to develop a sampling protocol for inaccessible areas before- hand, with full consideration of data analysis. We would not recommend that inaccessible areas be de- leted from restoration assessment and planning.

(2) The field sampling scheme that we developed and applied was adapted to the specific conditions



within our study area. The San Luis Rey River is a generally accessible study area that is geomorphically and biologically simple. In remote, complex basins it might not have been possible for us to meet our objectives for sampling intensity in the same manner.

(3) The classification or stratification of the reaches may have errors of omission or commission. Such errors will result either in excluding reaches from the field sampling or protection category that should have been included or including reaches for protection or field study that should have been excluded. The result is that some areas will be overlooked for protection or further study or field work will be conducted that might otherwise need not be done. We can guard against many of these errors by ensuring ade- quate verification using auxiliary data in the first stage of the study when we are classifying reaches.

(4) The use of cluster analysis to evaluate ecological conditions and trends in riparian communities worked well for plot data collected at the San Luis Rey fiver. In other studies of riparian settings, other methods have proven valuable (see literature cited in Harris & Olson, 1997), especially if environmental gradients are distinctive and communities are less disturbed. In general, the data analysis procedures used should be adapted to the setting, the data collected, and the skills and experience of study partic- ipants.

(5) Implementation of a conservation and restoration strategy in the San Luis Rey study area, or any other place where this approach may be used, depends on the political will of the people who control and use land in the floodplain. Protection of reaches recommended for protection requires regulatory and /or acquisition actions. Restoration within reaches recommended for improvement requires the design of specific projects. This study made no attempt to determine what ecological functions (e.g., wildlife habitat) should be emphasized in the San Luis Rey watershed; it assessed what the conditions are and what the restoration needs might be, relative to reference conditions found at the reach and community scales. It remains for the people that live in and regulate the watershed to determine the functions to be restored and the desired landscape and community patterns.

(6) The process outlined in this article is only a part of a comprehensive watershed analysis and the first step in riparian restoration planning. It serves to focus the effort for the site-specific planning and design that would be necessary to implement any restoration project. Implementation of projects would, at a minimum, require consideration of the following factors:

Overall size of the area to be restored Perimeter/area ratio Availability of water Connectivity to other riparian habitats Adjacent existing and proposed land uses Ownership Historical records on wildlife occurrence Presence or proximity of exotics Costs and requirements for continuous management Distribution and frequency of flood events Likely human and domestic animal activity

The utility of historical channel analysis for site-specific restoration planning is discussed in Kondolf & Larson (1995), with an illustration from the San Luis Rey River.

(7) An important element of implementation should be a program for monitoring river- and reach-scale conditions. Monitoring could be done, in part, through periodic aerial surveys. Permanent ground photo points and plots are appropriate for assessing specific restoration projects (Kondolf & Micheli 1995). Only through the monitoring and evaluation of restoration projects can we improve their design.

Acknowledgments

This research was funded by the U.S. Environmental Protection Agency's National Health and Environmen- tal Effects Research Laboratory, Western Ecology Divi- sion, in Corvallis, Oregon, through cooperative agree- ment CR819510-01-0 to the University of California at Berkeley. In the early phases of this project, Dr. Matt Kondolf, Assistant Professor of Landscape Architecture, University of California, Berkeley, provided thoughtful insights on research design. John LeBlanc, Marit Larson, and Sheila Kee participated in the design, data collection, and analysis. Field data were collected by Andrew Calkins and Jane Rogers. Suzanne Pierson, of OAO Cor- poration, produced Figure 1 under EPA contract no. 68- W5-0065, delivery order #19. Dr. Mary E. Kentula and Richard Sumner, U.S. Environmental Protection Agency, have been dedicated supporters. This article has been subjected to the Agency's peer and administrative review and approved for publication. Mention of trade names or commercial products does not constitute en- dorsement or recommendation for use.

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