Maria G6rska

Wysoczyzny Koninskiej. Pr. Inst. Geol. 48:147-162.

Woldstedt, P. & Duphom, K., 1974: Norddcur­schland und angrenzende Gebiete im Eiszeir­alrer. Stuttgart, Koehler: 500 pp.

Zandstra, lG., 1999: Plarenarlas van Iloordclijkekristaflijne gidsgesteC1l1ell. Backhuys Publish­ers, Leiden: 412 pp.

Landform Analysis, Vol. 4: 49-56 (2003)

Water erosion and supply of material for fluvial transport underepisodic surface flow conditions in the semi-arid zone on Boa Vista

(Cabo Verde)

Piolr Gierszewski

Polish Academy of SciencesInstiture of Geography alld Spatial OlganizationDepartmenr of Geomolphologyand Hydrology of Lowlalldsuf. Kopernika 19. 87-100 Torun

Jan Rodzik

Moria Curie-Sklodowska UniversityInstitute of Earth SciencesRoztoczanska Research Stationuf. Akademic:ka 19, 20-033 Lllblifl

LA Ab.wruct: This paper presents the effects of the natural conditions of the semi-arid zone on the tem­poral and spatial variability of production and supply of material for fluvial transport under conditionsof episodic surface flow. Based on measurements of water runoff, concentration of suspension and dis­solved material in the course of a one-hour rainfall, it has been determined that the catchments understudy have a considerable resistance to the effects of water erosion. Some protection is supplied by theablation pavement. which reduces rainsplash and slopewash and prevents now concentration. Consid­erably more erosion has been noted on unpaved roads concentrating water flow even on plateaux andgently inclined slopes. Obviously the potential for leaching rock material in short-duration flow islimited. Large proportions of the dissolved material carried away from the catchments arc salts of marineand aeoHan provenance. The high intensity of erosional processes in the early phase of runoff is evidenceof the role of physical weathering between rainfalls, which supplies the material for fluvial transport.

Key "'ord~': surface flow. waler erosion, suspended sediment, dissolved material, semi-arid, Caho Verde

48

Introduction

The Cabo Verde Archipelago is a group of 18volcanic islands, 4033 km2 in area. They aresituated in the Atlantic Ocean, 460 km west ofthe African coast, at the latitude of the southernextremity of the Sahara (Fig. 1). The eastern,older group of islands, including Boa Vista, prob­ably emerged from the ocean in the Miocene(Mitchell-Thome, 1972). The western islands ofthe archipelago are younger, a notion supportedby the contemporary vulcanicity on the islands ofFogo. The differences in the volcanic relief pro­vide further evidence of the different ages of theislands, and consequently of different lengths oftime of action of exogenous factors weatheringvolcanic rocks. The eastern islands (Sal, BoaVista, Maio) are nowhere higher than 436 m a.s.l.and have gentle relief whereas the others arehigher (from 774 to 2829 m a.s.l.) and much morediversified morphologically.

7 - Landfonn

Boa Vista, likc the remaining islands of thearchipelago, consists of volcanic rocks, mainlybasalts, phonolites and tuffs. Sedimentary rocks,up to several meters thick, represented by lime­stone, sandstones and conglomerates, occur onlyon marginal benches, whjch represent fanner sealevels (Mitchell-Thome, 1972). The island's rc­lief is dominated by vast flat pediments, glacisand marine terraces with gentle slopes. They arcdissected by valleys of temporary rivers. Volca·nic necks dominate the island's scenery.

The climate of the Cabo Verde islands isdetermined by their situation in the Atlantictropic zone, which is dominated by northeasterntrade winds. By contrast, the close vicinity of theAfrican continent causes an influx of dry andwarm land air-masses. The average annual rain­fall on the eastern flat islands of the archipel.ago does not usually exceed 150 mm (Klug,1973). More than 90% of the annual rainfall isconcentrated in the period from July to October.

49

Piatr Gierszewski, Jan Rodzik Water erosion and supply of material ...

Fig. I. Localization of study areaI - boundaries llf ca\ChmcnlJ. 2 - channel llf epirodic watercllursC5. J _ road, 4 _ altitudes m a.• .I.

o 0,5 1,0 km!L- ~! .....! CharaClerization of water runoff

The field research consistcd of observationand hydrological and geomorphological measure­ments of the effects of the heavy shower whichoccurred on 12 lb October 1999. They were car·ried out on a small slope fragment consisting ofa semi-natural catchment of an ablation basin,1,3 ha in area, and the catchment of an unpavedroad, 1,7 ha in area. Also included in the studieswere the outlets of the much larger neighbouringcatchments of the wadis Ribera de Abrolhos (52ha) and Ribera de Ervatao (612 ha) (Fig. I). Themeasurements of discharges. turbidity and elec­trical conductivity of water were carried out bothduring the shower and its aftermath. Consideringthe ephemeral character of the phenomenon, onlythe simplest of measurement methods were pos·sible. Height and intensity of precipitation markedby using calibrated container which have beenstuck oul in diffcrent rainfall's stages. Discharg­es in wadis' channels were assessed by the floatgauging mcthod and in slope's catchmcnts by thevolumetric method. Suspension's concentrationwas marked by the weight method and its grainsize by the laser technique (Fritsch Analyssene).

The surfacc water flow down the slopes of theCha de Ervatao plalcau, followed by runoff downthe channels of wadis were the result of a one­hour·long shower amounting 14 mm. For the firstfew minutes the rainfall was of low intensity.After that it was variable, but never exceeded0,5 mm/min.

Water flow from slope catchment areas start­ed after 15 minutes of rainfall. In the ablationcatchment it was overland flow, whereas in theunpaved road catchment it was concentrated.Rainsplash of material on the slope was clearlyreduced by the ablation pavement. AI the lime ofincreasing amount of flow the discharges, asregistered in the profile closing the slope catch­ments, amounted to 2, I dmJs- 1 in the ablationbasin catchment and 3,5 dmJs-1 in the unpavedroad catchment. The water flow culminationfollowed after about 50 minutes. The dischargevalues measured at the time were 2,5 dmJs- 1 and4,2 dmJs- 1 respectively. The moment the showerended, so did the sheet flow, and the concentrat­ed flow decreased considerably. Ten minuteslater, the discharges in the collective channels onthe slope amounted to 1,4 dmJs-1 in the ablationbasin catchment, and 2,1 dmls~l in the dirt road

Results of studies

The objcctivc, arca and methodsof studies

The main objective of the present paper is toassess the effect of cenain features of the naturalenvironment of the semi-arid zone on the spatialand temporal variability of production and sup­ply of material for fluvial transpon under episodicsurface overland flow conditions. In order to dothat the effects of a one-hour·long tropical showerof 14 mm were measured and analysed the con·ditions of water flow down the slopcs and runoffin the wadi channels were thereby evaluated. Thesources of supply of dissolved material and ofsuspended sediment for fluvial transport havebeen determined and the constraints of sedimentsupply in various flood phases was assessed. Thefield work was carricd out as part of the scien­tific expedition to Cabo Verde led by Pror. Dr.Elzbieta Mycielska-Dowgiallo (Gierszewski,2000).

The study area was a declivity in the Cha deErvatao Plateau, situatcd on the south-easterncoast of Boa Vista. This plateau situated at 60­70 m a.s.l, inclines gently (2-10°) towards theshoreline (Fig. 2). A series of tuffite sandstonesand calcareoites overlain by vulcanites stronglyweathered at the top overlies the basalt bedrock.The lateritic weathered material is covered bya basah ablation pavement. Its relief characteris·tics, geological structure and altitudes indicatethat the area in question is most probably anearly-Pleistocene (Sicilian) marine terrace(Mitchell-Thome, 1972). Lower benches arepresent on the declivity of the plateau at 40-50and 20-30 m a.s.1. Its rclief is diversified by ab­lation basins and deeply inserted (to 20-40 m),highly-branched valleys which may appropriate·Iy be described as wadis. That is a semi-arid area,as evidenced by the poverty of the herbaceousvegetation on the slopes. Only the more fertilefragment of the wadi floors arc vegetated by ex·tensive pulse crops and date and coconut palmplantations.

cesses arc; rainsplash, soil detachment, rilling andJUllying (Abrahams e, al.. 1994).

The shortage of water necessary to removesallS also rcduces the rate of chemical denudation.However, chcmical weathering can be quite con­siderable, as evidenced by the presence of min­eral duricrusts and the salinity of the upper partof the soil profile, caused by thc evaporation ofthe ascending ground water (Gat, 1980; Crabtree,1986; Nativ et 01.. 1997; Weisbrod er al., 2000).

--

.68

----------- .. ,, ,.. -" -...,,,,,--

Water erosion is considered to be the mostimportant relief transfonning factor in the geo­morphological landscapes of the semi-arid andarid zones (e.g., Langbein & Schumm, 1958;Saunder & Young, 1983; Woodward. 1995; De­tkov, 1998). That the high potential of erosion.due to intensive physical weathering processesand poor vegctation. is due not only to smallamount of infrequent rainfall, but also to thecharacteristics of the surface over which the run­off takes place is not generally appreciated(Nicholson, 1998). However, the low frequencyof rainfall is largely compensated by its torren.tial character. Even a single shower can causeconsiderable erosion.

Both the considerable surface sealing of desenareas, due to the consolidation of soil aggregatesby rain drops, and the occurrence of various kindsof mineral duricrusLIi obstruct the infiltration ofrain water (Romkens et al., ] 990). Under suchconditions practically the entire runofT becomesoverland flow, and the predominant erosive pro-

•

DEl 2

CS] 3

0,

•

The weakening of the trade winds towards theend of summer accounts for an influx of warmand humid equatorial air masses, the result ofwhich are the prevalent torrential short·durationrains. Data from the area of Praia (Santiago[sland) inform about the frequency of rainfall'sincidence. In the years 1980---1987 notified 63rainfall events there. However only 29 timesoccurred falls above 10 mm which, in theseconditions, cause erosion (Mannaerts, Gabriels,2000). However, since 1968 the rainfall condi­tions have deteriorated considerably. The rainyseason, which before that date characteristical­ly lasted five months, contracted to only twomonths, and the total annual rainfall decreasedby nearly 50%. Subsequently catastrophicdroughts, lasting 2-3 years in succession, arenot unusual (Ferreira, 1987). The progressivedesertification of the islands, together with thetorrential rains, cause intensive erosion. This iscurrently the most important factor in the mod­eling of the islands' relief (Ferreira, 1996).

50 7' 51

Piotr Gierszewski, Jan Rodzik Water erosion and supply of material ...

Fijt. 4. COlleenlnltion of suspension (A) and dissolvcd material (B) in successive minutesof runoff

120

100

180

• R.de Ervatao - 5'

0 R.de Ervatao - 35'

• R.de Abrolhos - 5'

0 R.dc Abrolhos - 15'

0 R.de Abrolhos - 30'

W slope-road - 50'

• slope-basin - 50'

0,00016 mm

Discussion of results

Observations have demonstrated that rainfall,even of comparatively low intensity and shortduration, initiates water surface flow on the studyslopes. The time lag of the flow relative to thebeginning of rainfall (c. 15 minutes) was presum­able due to the low rainfall intensity in its earlystage but it could also be associated with waterretention in the covers of weathered material on

The mineralization of water outflowing fromthe slope catchment was similar to that from theAbrolhos catchment. The water of the Ervalaocatchment were more highly mineralized (Fig.48). Its eastern part, drained by the Calheta, ismade up of leached marine sediments containinga large proportion of carbonate rocks.

minutes

0,00010.005

o 5 10 15 W H ~ 35 ~ 45 ~ 55 ~ '5 ~

0,01

,-----------------,'00

~­0,~"-.

0+--+--+---+-->---+-+----<-1--+--+--+--+---+-->--'-+

25

'0/./~

~ IS / "E

'":e '0

5

50

75

Fig_ 6. Variation in specific conductance (SEC) compared with water stagc (H) in mouth segmentof Ribeira de Abrolhos

,,o+----""==--.---~--~--___j

%100,-----------~ ",_---.

25

Fig. 5. Cumulation curves of graining of suspension in successive minutes of runoff

It was then 188 ~S cm-I, i.e. four times as muchas that of the rain water (mean of the total show­er 55 ~S cm-I). As the water level increased, itsconductivity decreased. After 15 minutes, short­ly after the flood culmination, it decreased by 50)1S cm· l

. In the course of the following 20 min­utes, the conductivity of the water did not signif­icantly change, after while it gradually increasedthrough only slightly. In the flow cessation phase,water conductivity increased presumably due toevaporation.

In order to dctermine water turbidity, mea­surements of \OIal mineralization were also car­ricd out in the samples taken (Fig. 48). The lim­ited number of samples analyzed makes it diffi­cult to define the progress of the dissolvedmaterial outOow. However, a clear tendency ofvariability can be seen, as noted in the plot of theconductivity in the Abrolhos catchment.

B

5a15'

15'

5'

5'

Dissolved material outflow

The role of the particular sources of supply ofdissolved material and the variation in the timeintensity of its outflow from the catchments undersrudy is elucidated by the measurements carriedout at the mouth of the Abrolhos (Fig. 6). Thehighest specific electrical conductivity of water(SEC) was found there at the start of the outflow.

A

conditions. The highest turbidity occurred in tbeinitial phase of water runoff (0,86 g dm-l), thelowest at the end of the flood (0,37 g dm-l).Suspension transport in the Ervatao wadi pro­ceeded in the same way (Fig. 4A). However, themore morpbologically diversified catchment areaof that wadi supplied considerably more materi­al for fluvial transport, which was reflected in themuch higher concentration of the suspensioncarried down the wadi channel. The greater en­ergy of the flood waters of the Ervatao was alsoreflected in the increased percentage of the siltfraction in the total mass of me transported sed­iment, which at the start of the runoff was asmuch as 46%. The suspension of the Abrolhosconsisted mainly of clay with a subsidiary silt(Fig. 5).

35'

35'

,

5'

5'

mJldm

En'1llao AbroLbos ro•• basin

ErvalllO Abrolhos ro•• basin

11 I

120

100

80

60

40

'0o

180

160

The measurements of suspension transportintensity in the catchments of a semi-naturalablation basin and a road ditch were carried outin tbe falling limb of the

gdm·J

flood. The amount of suspcn- 6

sion lransported in that phaseof the flood was already much 5smaller, although there were 4essential differences betweenthe catchments under study. 3

The turbidity value of thewater flowing down the road 2

ditch was nearly ten times asgreat (3,12 g dm·l) as that inthe ablation basin catchment 0(0,36 g dm·') (Fig. 4A). Thesediment transported in tbatflow phase was representedmainly by the clay fraction(Fig. 5). In the measurementsection of the ablation basin, 140

that fraction accounted for99,63% of the transportedsediment, and in the catch­ment of the road ditch,82,65%. The remaining partof the transported materialwas silt.

The concentration of thesuspended material in thelower segment of the Abrol­hos wadi varied with the flow

Sheet wash and suspended outflow

catchment. Aftcr another five minutes, the run­off from the slope catchments was terminated.

Water runoff in the upper reaches of the wadisstarted the moment the showcr finished. Theflood wave front appeared at the mouth of theAbrolhos wadi channel half an hour after thetennination of the shower and in thc Ervatao wadichannel five minutes later. The channels, com­pletely dry up till then, rapidly filled with turbidwater (Fig. 3). The flood was characterized by avery short, only 5-10 minutes long, phase ofwater level increase followed by a prolonged fall.The discharge at the time of flood culminationwas estimated at ca 100 dmls· 1 in Abrolhos andat 2000 dmls- I in Ervatao. Water runoff from thesmaller Abrolhos catchment was terminated af­ter 40 minutes, that from the Ervatao catchmentstopped after 2 hours and 10 minutes. Thus, thetotal duration time from the start of watcr flowfrom the slopes to the tcrmination of the runoffin the wadi channels was 3,5 hours.

52 53

Piotr Gierszewski, Jan Rodzik

slope. Ccrtainly a much higher retention capac­ity characterized the alluvia filling the wadi chan­nels. The fact that runoff appears there aftcr onlyonc hour's rainfall is evidence of intensive infil­tration of water into the alluvia.

The turbidity values and the textural charac­teristics of the transported suspension point toa diversity of resistance to water erosion of theslope catchments under study. Despite havinga smaller catchment area, the road flow showeddischarges nearly 70% higher than those of theablation basin catchment flow. The higher dynam­ics of the hydrological process in the road catch­ment is reflected in the characteristics of thesuspension load, which was characterized byconcentration values ten times at of the mhercatchment areas and by a higher percentage ofcoarser-silt fraction in the transported material,the greater amount of material transported fromthe road catchment must be associated with ero­sion of the old and the curremly used unpavedroad. Repeated showers were conductive to thedevelopment along the line of the old road; of agulley this was area I m2 in cross section. Thisdeveloped from an evorsion hole 8 m) in volumeat the edge of the plateau (Fig. 7). The consid­erable role of unpaved roads in supplying weath­ered material to river channels in other climatezones has been emphasised by a number of au­thors (e.g. Froehlich, 1995; Fransen et al., 200 I;Madej, 2001).

The lesser erosion in the semi-natural ablationbasin catchment is, to a large extcnt, due to theprotective function of the ablation pavcmenlwhich covers the surface of the plateau and theslopes of Cha de Ervatao. Certainly, the stonycovers occurring on the terrain surface effective­ly reduce the water erosion (Poesen et al., 1994).Their prescnce considerably reduces rainsplash.enhances infiltration and percolation and causesincreased frictional coefficients. This eventuallyleads to both a reduction in the volume and aslowing down of the overland flow, and conse­quently of erosion. According 10 Poesen et af.(1994), even a 10% cover of the terrain by stonesconsiderably reduces erosion. In the case of theCha de Ervatao Plateau and declivity, that per­centage is much higher - as much as 60%. How­ever, in what seem to be a seemingly homoge­neous surface covered with ablation pavementcertain places appear to be more liable to erosionthan others. Erodibility clearly both on the small­er number of stones accumulated and on theirarrangement. According to Posen and Ingelmo­Sanchez (1992), boulders embedded in hardenedsurfaces no longer constitute such effective pro-

54

tection against erosion. In the case in qucstion,these can be the various types of duricrust, whoseconsiderable development on Boa Vista has beenrecorded by Mitchell-Thome (1972), and compactdeluvial covers occurring on flat slopes where,according to the authors' measurements, the ef­fects of erosion arc particularly high, particularyalong unpaved roads.

According to Frostick et al. (1983) and Lavccet of. (1991), a high concentration at the verybeginning of the flood is a characteristic featureof suspension outflow down channels of wadis.At that time, the coarser fractions constitute asignificant proportion of the transported materi­al. Although the number of samples analysed issmall, the results agree with those them to bytypical of suspension outflow down channels ofepisodic water courses have been validated by theresults obtained in the Abrolhos and Ervataowad is.

The size of the transported particles and thcfact that the maximum concentration appears atthe beginning of the flood, both indicate the slopesurfaces as the principal source of material sup­ply. The sediment washed down from the slopesrepresents similar textural characteristics to thatof the suspension load present in the wadistreams. Their channels, in filled-up with grav­elly-sandy alluvia can only affect the concentra­tion and the granulometric composition of thetransported suspension at the time of floods withlower energy of water flow to a small extent. Theprovenance of the material transported in thechannel illustrates the close relationship of thefluvial and slope systems in the denudation sys­tem under study. In the light of the above state­ment it seems cvident that the Ervatao catchment,being morphologically more varied and charac­terized by a denser valley nctwork, suppliedconsiderably more material for fluvial transportthan its neighbours.

The general salinity of the waters, whichdrained from the catchment arcas under study atthe time of the rainfall flood studied is high. Thesource of salt here is not only the basalts but alsothe easily leached carbonate marine sediments,which occur in the Ervatao catchment area.A considerable proportion oftbe tOlal mineraliza­tion of water is explained by non-denudationalcomponents. The largest part is played by marincsalt supply, which resulls from the situation of thecatchment area. The oceanic provenance of therainfall water supplying the islands, as indicatedby the high value of the a-fCr index, has beendemonstrated by Louvat & AlIegre (1998). Ac­cording to these authors, this kind of salt supply

into the system cannot be ignored when assess­ing the denudation balance. Owing to the closein the area under study vicinity of lhe ocean themarine aerosols deposited on its surface in rain­less dry periods arc an important source of salt.Also thc dryness of Boa Vista's climate and theresulting aeolian salinity of the environmentcontribute to an increased concentration of thematerial dissolved in the surface waters. Theincrease in total mineralization of rain watersflowing down the terrain surface is also relatedwith the dissolving of salts contained in the ae­olian dust which to present on the surface of thecatchment area (Gierszewski, 1998). The salini­ty of the upper part of the soil profile resultingfrom evaporation of soil moisture is clearly ofgreat importance in this case (cf. Gat, 1980; Naivet al., 1997; Weisbrod et al., 2000). The highestsalt concentration in the water outflowing fromthe catchments under study is therefore to beexpected at the beginning of the rain season asa whole and each particular rainfall episode.

The general conditions affecting also in theextent of total mineralization of the surface walerin (he semi-arid zone in catchments within the in­fluence range of the ocean are fully reflected in theresults of studies carried out on Boa Vista. In theinitial phase of water outflow, salts of marine andaeolian provenance are dominant in the solutionload removed from the catchments. However, theirsupplies are soon exhausted as is indicated by thepronounced decrease in dissolved material concen­tration after only a few minutes of flood, and oftenbefore its culmination. Thereafter the water min­eralization does nOt change to any significantextent. A similar relationship has been observed inthe Judea desert (Levee et al., 1991).

Conclusion

The results of our studies make it possible toidentify water erosion as the dominant process inthe modeling of the contemporary relief of thestudy area. The high intensity of erosional pro­cesses in the initial runoff phase emphasizes therole of physical weathering in the periods be­(ween rainfatls as the factor responsible for thepreparation of material for fluvial transport(Abrahams et al., 1994). The torrential showerswhich occur with varying frequcncy are evident­ly quite capable of carrying away most of theweathering products.

It must, however, be pointed out that thecomparatively gentle natural slopes on Boa Vis­ta are very stable despite of the sparse vegetation

Water erosion and supply of material ...

cover there. The protective factor here is theablation pavement, which reduces rainsplash andsheet wash and minimises flow concentration.Under natural conditions. erosion is limited to adeepening of the upper parts of valleys and aretreat of the wadis heads. In the wadi floors,there arc usually channels with a marked prev­alence of aggradation. In contrast, the un pavedroads concentrate water flow even on the plateauxand very slightly inclined slopes.

Owing to obstruction of water infiltrationwhich is the present of soil salinity (Lavee et al.•1991), or the occurrence of compact diluvialcovers and duricrusts, or the shallow profile ofpermeable deposits. the potential for leaching ofrock material in these Short-lasting flows is ex­tremely limited. A considerable portion of thedissolved material carried away from lhe catch­ments represents salts of marine and aeolianprovenance, and their supplies are concentratedon the terrain surface and in the upper part of thesoil profile.

The characteristics of transport of dissolvedmaterial and suspension in the wadi channelssuggest a close relationship between the fluvialsystem and the slope system in the denudationsystem under study. Most of material washeddown from the slope is carried away down thewadi channels to (he sea.

Acknowledgments

The authors wish to acknowledge their grat­itude for financial support to the directors of theInstitute of Geography and Spatial Organizationof the Polish Academy of Sciences, to the author·ities of Maria Curie-Sklodowska University inLublin and to the City Office in Toruli.

References

Abrahams, A.D., Howard, A.D. & Parsons, AJ.,1994: Rock-mantled slopes. In: A.D. Abra­hams & AJ. Parsons (Eds.) Geomorphology ofdesert envirOnments. Chapman & Hall, London:173-212.

Crabtree, R. W., 1986: Spatial distribution of so­lutional erosion. In: S.T. Trudgill (Eds.) Soluteprocesses. John Wiley & Sons, Chichester:329-36 J.

Dedkov, A.P., 1998: Erosion in the arid zones.Geomorphology 4: 3-12 (in Russian).

Ferrcira, 0.8., 1987: La crise climatique actuelledans I'archipel du Cap Vert. Quelques aspects

55

Piotr Gierszewski, Jan Rodzik

du probleme dans rile de Santiago". Finister­ra-Revista Porwguesa de Geografia 22 (43):113-152.

Ferreira, 0.8., 1996: Water erosion in the CapeVerde Islands: factors, characteristics and meth­ods of control. In: O. Slaymaker (Eds.) Geo­morphic hazards. John Wiley & Sons, Chich­ester: 111-124.

Fransen, P.J.B., Phillips, CJ. & Fahey, B.O.,200t: Forest road erosion in New Zealand:Overview. Earth Surface Processes and Land­forms 26 (2): 165-174.

Froehlich, W., 1995: Sediment dynamics in thePolish Flysch Carpathians. Ill: 1.0.L. Foster,A.M. Gurnell & B.W. Webb (Eds.) Sedimentand water quality in river catchments. JohnWiley & Sons, Chichester: 453-461.

Frostick, L.E., Reid, l. & Layman, J.T., 1983:Changing size-distribution of suspended sedi­men! in arid-zone flash floods. IlIlernationalAssociation of Sedimentologists, Special Pub­lication 6: 97-106.

Gat, J.R., 1980: The relationship between surfaceand subsurface waters: water quality aspects inareas of low precipitation. Hydrological Scienc­es - Bulletin - des Sciences Hydrologiques25(3, 9): 257-267.

Gierszewski, P., 1998: Og61na charakterystykacech chemicznych w6d powierzchniowyehCentralnej i Zachodniej Mongolii. Zeszyty IG­iPZ PAN 52: 37-50.

Gierszewski, P., 2000: Wyprawa naukowo-badaw­cza geograf6w na Wyspy Zielonego Przyl~dka.

Przeglqd Geograficzny 72 (3): 332-337.Klug, 11., 1973: Die Inselgruppe der Kapverden,

Schriften des Geographischen ["stituts derUniversitiit Kiel 39: 169-204.

Langbein, W.B. & Schumm, S.A., 1958: Yield ofsediment in relation to mean annual precipita­tion. Transactions of the American Geophysi~

cal Union 39: 1076-1084.Lavee, 1·1., Imeson, A.C., Pariente, S. & Benyami­

ni, Y.. 1991: The response of soils to simulat­ed rainfall along a c1imatological gradient in anarid and semi-arid region. Catena Supplement19: 19-37.

Louvat, P. & Allegre, CJ., 1998: Riverine ero­sion rates on Sao Miguel volcanic island,

56

Azores archipelago. Chemical Geology 148.177-200.

Madej, M.A., 2001: Erosion and sediment deliv­ery following removal of forest roads. EarthSurface Processes and Landforms 26 (2): t75­190.

Mannaerts C.M., Gabriels D., 2000: Rainfallerosivity in Cape Verde. Soil & Tillage Re­search 55: 207-212.

Mitchell·Thome, R.C., 1972: Outline of theGeology of (he Cape Verde Archipelago. GeolRdsclr. 61 (3): 1087-1109.

Nativ, R., Adar, E., Daban, O. & Nassim,l., 1997:Water salinization in arid regions - Observa­tions from the Negev desert, Israel. Journal ofHydrology 196 (1-4): 271-296.

Nicholson, S. E., 1998: Desert hydrology. In: R.W. Herschy & R.W. Fairbridge (Eds.) £ncycJo~

pedia ofHydrology and Water Resources. Klu­wer Academic Publishers, Dortrecht: 176--183.

Poesen, J. & Ingelmo-Sanchez, F., 1992: Runoffand sediment yield from topsoils with differentporosity as affected by rock fragment cover andposition. Catena 19: 451-474.

Poesen, J. W., Torri, D. & Bunte, K., 1994: Effectsof rock fragments on soil erosion by water atdifferent spatial scales: a review. Catena 23:141-146.

Romkens, MJ.M., Prasad. S.N. & Whisler, ED..1990: Surface sealing and infiltration. Ill: M.G.Anderson & T.P. Burt (Ed.f.) Process SlIIdies ill',ills/ope hydrology. John Wiley & Sons. Chich­ester: 127-172.

Saunders, I. & Young, A., 1983: Rate of surfaceprocesses on slopes, slope retreat and denuda­tion. Earth Surface Processe.f alld Landforms8: 473-501.

Weisbrod, N., Nativ, R., Adar, E.M. & Ronen, D.,2000: Salt accumulation and flushing in unsat­urated fractures in an arid environment. GroulldWater 38 (3): 452-461.

Woodward, J.c., 1995: Patterns of erosion andsuspended sediment yield in MediterraneanRiver Basins. In: I.D.L. Foster, A.M. Gumcll& B.W. Webb (Eds.) Sediment and water qual­ity in river catchments. John Wiley & SonsLtd., Chichester: 365-389.

Fig. 2. Fragment of slope surf"ce of thc Cha de En"lao plateau

Fig. 3. Mouth segment of the Ribeira de Ervatao wadi

Land/orm Analysis, Vol. 4: 57-64 (2003)

Kazimierz Krzemieri

Jagielloflian UniversityIlIstitllte of Geography and Spatial Managementul. Grodzka 64, 31-044 Krak6w, Poland

The Czarny Dunajec River, Poland, as an example of human­induced development tendencies in a mountain river channel

Abstract: The Czamy Ounajec is a typical river origmating in high mountains (the Western Tatras). Overtbe entire Qualernary era tbe river laboured carrying material away from the Tatras and depositing it inthe form ofltypical brtided channel at their foot. Atlbe cnd of the 19· century. river management projectsand quarrying operallons located directly in the very chlnnel set off I rejuvenltlon process thal was furtheraccelerated at the end of 196Os. The activitIes resulted in the damaged several sectIons in their naturalform and considerably deepened the channel. Measures taken to restrict the Imount of material enteringthe Czorsltyn Dam have largely failed. From the geomorphologic and enVironmental points of viewI continued transformation of the Czamy DunaJee river ebannel should be regarded as highly ad\·erse.

---LA

Fig. 7. Evorsion hole in the cUllmg of the unpa\cd road

Kcy "'ords: mounlain river channels. fluvial processes. human pressure. Western Carpathiao Mts.

Introduction

The contemporary diversification of riverchannels is a result of long-tenn processes cov­ering the entire catchment basins. As active chan­nel sections indicate the current stage of a riverchannel evolution, a description of the entire riverchannel system helps understand the principles ofhow it works (Choeley, Kennedy, 1971). Infonna­tion on the entire channel systems is mainlysupplied by field research, supplemented by mapsand aerial photographs (Mosley, 1987; Tbome,1998; Kamykowska et al., 1999). However, asmost of the research has focused only on select­ed river sections, little is known about riverchannel systems at the scale of entire mountainranges or even larger catchment areas.

Similarly, treated as a whole, the upper Vis­tula-catchment system is unquestionably under­researched. Meanwhile, as several Carpathianrivers are subject to management schemes andtheir channels dug for building aggregatcs, theirsystems have not been properly investigated. Theresearch carricd out so far shows that alterations

made to onc river section can lead to changes,which are difficult to predict in others (Osuch,1968; Klimek, 1983). In order to determine theircurrent status and to try to predict the develop­ment tendencies (Wasson et al., 1993; Chelmickiand Krzemieil, 1999), therefore, it is very impor­tant to evaluate the cntire systems. Channel struc­tures should be evaluated first and only then canchanges be made. The Czamy Dunajec river isa very wonhy object of such an investigation, asits river channel has becn subject to intensivehuman-induced transformation involving rivermanagement and aggregate extraction, especial­ly after the Second World War. These acti\litiesare still being carried out despitc the official dec­larations of the relev80l authorities that no fur­ther river management work is planned and theextraction of the material from the river beinglegally prohibited.

The research project conducted in the CzarnyDunajec catchment basin was aimed at under­standing the structure of the channel system anddemonstrating thc natural and human-inducedcauses of its transformation.

• _ Landform 57