mississippi river diversions and coastal restoration
DESCRIPTION
MISSISSIPPI RIVER DIVERSIONS AND COASTAL RESTORATION. Christopher M. Swarzenski United States Geological Survey, La Water Science Center, Baton Rouge. Some CONTEXT. SOIL IS THE PRIMARY METRIC TO EVALUATE EFFECTIVENESS OF ANY RESTORATION APPROACH - PowerPoint PPT PresentationTRANSCRIPT
MISSISSIPPI RIVER DIVERSIONSAND COASTAL RESTORATION
Christopher M. SwarzenskiUnited States Geological Survey, La Water Science Center, Baton Rouge
SOME CONTEXT
SOIL IS THE PRIMARY METRIC TO EVALUATE EFFECTIVENESS OF ANY RESTORATION APPROACH
- Quality: resists tendency to erode, such as from tides and storm surge
- Quantity: accretes vertically fast enough to keep pace with relative sea level rise (subsidence+eustatic sea level rise)
NUTRIENT REQUIREMENTS OF COASTAL MARSHES
NO peer-reviewed science that an insufficient supply of nutrients has contributed to wetland loss in coastal Louisiana or anywhere else in the world
NO peer-reviewed science that adding nutrients helps or has helped reduce rates of marsh loss in coastal Louisiana or anywhere else in the world
CHENIERPLAIN
MISSISSIPPI DELTAPLAIN
ActiveAtchafalaya River
ActiveBird’s foot DeltaInactive
New Orleans
WITH MINERAL MATERIAL,Recently deposited mudWest Bay, August 2011inorganic, heavy, inanimate
WITH ORGANIC MATERIALEroding shoreline,
Lake Salvadorhighly organic, light, “living”
Building and Maintaining Delta Wetlands
ACTIVE DEPOSITION
IN SITU ACCRETION
TYPES OF RIVER DIVERSIONS
I. WATER-QUALITY
a. “rejuvenation” e.g. Caernarvonb. “maintenance” e.g. Davis Pond
examples Caernarvon, Davis Pond, Terrebonne basin
II. MINERAL SEDIMENT (land-building)
examples include Wax Lake Outlet, West Bay, Bird’s Foot delta
“REJUVENATION”
PurposeTransform a degraded (marsh) soil into something less degraded => higher-quality soil
Constituents in River Water- fresh water (reduces salinity)- fertilizers (nitrate-N, orthophosphate) - sulfate- herbicides- but very few clays and silts
What we might hope for when we “rejuvenate” a marsh
Water-quality diversion
Existing degraded S. patens marsh soil
Soil of S. falcata marsh, typical of new salinity regime
“The diversion isn't filling the marsh with sediments on a grand scale”. But the effect of the added river water—loaded as it is with fertilizer from farm runoff—is plain to see. "It turns wetlands hanging on by the fingernails into something quite lush"
“Those nutrients are necessary for nourishing the marsh environment”
SCIENTIFIC CONSENSUS
“Two diversions on the Mississippi are already demonstrating how well diversions can work to restore wetlands. Although these are freshwater diversions (meaning they pull water off the top of the river rather than the bottom, where heavier sediment loads are) and thus do not provide a lot of sediment to the wetlands, they are still beneficial”
REALITY: Water quality diversions appear to compromise soils
I. Swarzenski et al 2005, 2008, freshwater Panicum marshesChronic passive inflow of Lower Atchafalaya River water since 1950s coincided with massive conversion of freshwater Panicum marsh to open water and degraded marsh, soils more decomposed than reference marshes, observation, linked to river water influx Biogeochemistry 90, p. 49-63.
II. Kearney et al. 2011 brackish S.patens marshesCaernarvon, White Ditch and Pte a la Hache Post Katrina, areas within outfall were more eroded than adjacent reference marshes at all three controlled diversions; no data on causes, remote sensing, correlation Geophysical Research Letters 38 L16405
III. Deegan et al, 2012, New England S. alterniflora salt marsh9 years of nutrient additions: plants produced fewer roots, organic matter decomposed more rapidly and creek banks collapsed, experimental, causative Nature 490, p 388-392.
I. Demise of Penchant Panicum marshes (1955-1998)
pre 1955 1998
• Rain is primary sourceof fresh water• Flow east to west• Solid expanse ofP. hemitomon
• River is primary sourceof fresh water• Flow west to east• Loss of > 60% of P. hemitomon marsh
Swarzenski et al Biogeochemistry 2008
Three to six months of passive river water influx for 40+ years (Lower Atchafalaya River example)
Soil Quality: with long-term (40+ years) river water influx decomposition appears enhanced (same plant community, Panicum hemitomon)
- No difference in year-end standing biomass- No difference in accretion rates-More organic matter in marsh receiving river water
BUT:- Soil is much more decomposed
- links to river water include sulfate, nitrate, alkalinity, pH
River water subsidyReference
Swarzenski et al Biogeochemistry 2008
OBJECTION: NUTRIENTS GOOD NUTRIA BAD
Nutria are the problem, herbivory is stimulated by nutrient enriched plant material, nutria multiply out of control and marsh dies off
Photo from Holm et al 2011, LPBF
River Reference/Rain
SEPTEMBER ABOVE-GROUNDSTANDING BIOMASS (n=4)
grams m2
504(30) Total 538(31)
337(27) Panicum 441(31)
SOIL (n=3)
14.5 12.8
225 187
Stock (kg C,.5 m by 1 by 1 m)
Accretion, (g CPer m2 per year)
Swarzenski et al Biogeochemistry 2008
Outfall
Hydrologicbarrier
Caernarvon
II. Controlled diversion at Caernarvon, post Katrina
Kearney et al 2011, Geophysical Letters
Outfall Marsh
East Controlin direct path of Katrina
MARSHES POST KATRINA WITHIN OUTFALL AND ON EAST SIDE OF DELACROIX RD
Kearney et al 2011, Geophysical Letters
OBJECTION: Pattern of loss is random, or energy was concentrated on west side, but not on east side of Delacroix Road
PRE KATRINA
POST KATRINA
Kearney et al 2011, Geophysical Letters
“ By contrast, when nutrients were added, the authors observed that plants produced fewer roots, organic matter decomposed more rapidly and creek banks collapsed, leading to wider creeks and less vegetated marsh, with narrower bands of S.alterniflora”
III. DEEGAN ET AL, 2012, Nature
OBJECTION:
Deegan et al 2012, Nature
New England salt marshes are different
- Macrotidal environment (5-7 ft tides)
- Ice formation (erosion)
1. WEAKENED SOILS ARE THE COMMON THREAD
- Correlational (field observations, remote sensing) and experimental studies in different marsh types from Atlantic and Gulf Coasts point to similar outcomes, weakening of soils: in areas with nutrient enrichment, soils are more decomposed and erode more readily during daily tides and during storm events
- excessive flooding could also contribute to weakening of soil
2. ABSENCE OF EVEN A SINGLE STUDY SUGGESTING SOIL IS GAINING SHEAR STRENGTH WITH RIVER WATER SUBSIDIES
Conclusions (Water-quality diversions)
TYPES OF RIVER DIVERSIONS
I. WATER-QUALITY
a. “rejuvenation”b. salinity control
examples Caernarvon, Davis Pond, Terrebonne basin
II. MINERAL SEDIMENT (land-building)
examples include Wax Lake Outlet, West Bay, Bird’s Foot delta
Scientific consensus is that massive sediment diversions off of mainstem river, in mid-Barataria Basin, are key to restoring coast
What is a massive sediment diversion?
50,000-80,000 cfs controlled ? 1-2 months per year?? 3-4 months per year??
>250,000 cfs uncontrolled??
Is mid-Barataria Basin a suitable location for massive sediment diversions in terms of initiating processes for a sustainable coast?
EVIDENCE THAT SEDIMENT DIVERSIONS OFF THE MAIN STEM WILL WORK: WAX LAKE OUTLET
"To say that there's not thriving vegetation there with the exact same water that's in the Mississippi River is just a fundamentally flawed statement,"
NAS, NWF, EDF, http://www.youtube.com/h?v=i-sJFoM-Lsc
Wax Lake Delta: a diversion that works
accretion (cm/year)
Min
eral
sed
imen
ts,
kgin
top
24
cm p
er m
2
0 1 2 3 4 50
50
100
150
200
250
Birds Foot
LAR/WLO
Significantly more mineral sediment is deposited in the Bird’s foot delta wetlands than in the Wax Lake/Lower Atchafalaya River wetlands
THE BIRD’S FOOT DELTA IS NOT BUILDING LAND
WAX LAKE OUTLET IS NOT A GOOD ANALOG FOR SEDIMENT DIVERSIONS OFF OF THE MAIN STEM MISSISSIPPI RIVER
SYNERGY Combined 1/3 of the Mississippi River flow and about ¼ of the sediment load
PLATFORM previous delta building occurred several thousand years ago, many years of marine reworking has built a firm platform to support weight of new mineral deposits
SUBSIDENCE AND COMPACTIONRates are probably much lower than in Barataria and Breton; Holocene layer is much thicker in latter area
Conclusions
I. WATER-QUALITY
a. as “rejuvenation” problematic, appears to exacerbate marsh loss; adaptive management calls for refinement of approach, detailed studies before being implementedb. as “maintenance” works in short term, but only needed occasionally
II. MINERAL SEDIMENT (land-building)
- specifics are needed: size, location, key uncertainty is a better understanding of weight bearing capacity of underlying strata
SHOULD DIVERSIONS PLAY A ROLE IN COASTAL RESTORATION
Emphatic yes, QUESTION IS HOW?
Clarity is needed on what the expectations are for main-stem river diversions as proposed in Master Plan and then a transparent scientific debate whether these expectations are reasonable given our geological and ecological knowledge.