Great Basin Naturalist
Volume 53 Number 2 Article 3
6-4-1993
Response of a Sonoran riparian forest to a 10-year return flood
J. C. Stromberg Arizona State University, Tempe
B. D. Richter The Nature Conservancy, Boulder, Colorado
D. T. Patten Arizona State University, Tempe
L. G. Wolden Arizona State University, Tempe
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Recommended Citation Stromberg, J. C.; Richter, B. D.; Patten, D. T.; and Wolden, L. G. (1993) "Response of a Sonoran riparian forest to a 10-year return flood," Great Basin Naturalist: Vol. 53 : No. 2 , Article 3. Available at: https://scholarsarchive.byu.edu/gbn/vol53/iss2/3
This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Great Basin Naturalist 53(2), pp. 118-130
RESPONSE OF A SONORAN RIPARIAN FOREST TO A IO-YEAR RETURN FLOOD
l l J. C. Stromberg , B. D. Richter", D. T. Patten" and L. G. Wolden
3 1 ABSTHAGr-In March 1991 a 10-year return flood (368 m s- ) occurred in the Hassayampa River, a perennial stream (0.1 m3s-1. base flow) within the Sonoran Desert. Depth ofthe floodwater ranged from 2.64 + 0.20 m (mean + SO) near the stream to 0.47 + 0.31 m in the highest floodplain wne (Prosopis forest). Flow velocity was 1.7 + 0.6 m s-1 and 0.9 + 0.4 III s~l in these same zones. An average of 8 cm of sediment was deposited on the floodplain, with maximum deposition (to 0.5 m) on densely vegetated surfaces 1-2 m above the water table. Native riparian vegetation showed resistance and resilience to the flood disturbance. Plants on high floodplains (e.g., Prosopis velutina trees and saplings, and Populus fremontii and Salix goo
Key uxJrd~': riparian vegetation, flood flow, disturbance, Populus fremontii, salix goodrungii,floodplai-n aggradation, resilience.
Flood flows have been said to be the "prin Decreased flooding is often a greater perturba Cipal driving force responsible for the existence, tion to riparian floodplains than is flooding productivity, and interactions ofthe major biota (Sparks et aI. 1990), as indicated by the substan in river-floodplain systems" (Junk et aI. 1989). tial vegetational changes that occur when rivers With resped to floodplain vegetation, flood are dammed and flooding is suppressed (Reily flows play an integral role in the dynamics of and Johnson 1982, Pautou et aI. 1991). Dam seed dispersal, plant establishment, and species construction may result in increased riparian replacement patterns; maintenance of species acreage in sediment deltas at upstream ends of and "patch" diversity; and nutrient cycling and reservoirs (DeBano and Schmidt 1990), but al productivity (Bell 1974, Johnson et al. 1976, tered flow and sedimentation patterns down Smith 1980, Long 1982, Reichenbacher 1984, stream can result in decreases in plant Kalliola and Puhakka 1988, Lisle 1989, Skog establishment rates and loss of riparian forests lund 1989, Stromberg et aI. 1991). Riparian (Rood and Mahoney 1990, Howe and Knopf forests in arid regions are particularly depen 1991). dent on flooding because forest regeneration The size differential betweenbase flows and often depends on periodic inundation ofother flood flows of a given recurrence intelVal is wise dry floodplain surface soils (Hughes 1990). greater in arid regions than in humid regions
I O'"ter f",- 1~llvinmlllent,,1 f>tudie., ArimlHl f>tllte Univer~ity. Tempe. Ari7~>1Hl fl52i:l7-32lJ.. ZTh" N"lll,",' C""."rvmK')'> B<"lhkr> Cvl"rHdo 80302.
118 1993] RESPONSE TO lO-YEAR RETURN FLOOD 119
(Graf 1988). Large desert floods can cause sub return flood (193 m's-I) in AU9;':'t 1988 and and strate erosion and plant removal in systems in a 2-year return flood (68 m's- ) in July 1990. which stabilizing vegetative cover has been re duced by cattle grazing, base-flow reduction, or STUDY AREA other factors (Platts et al. 1985, Gordon-Ish Shalom and Gutterman 1989, Stromberg and The Hassayampa River lies within the Gila Patten 1992). This occurred in large scale in late watershed ofcentral Ariwna's Basin and Range nineteenth-century Arizona when large floods Province and drains portions of tl,e Bradshaw, on denuded floodplains and watersheds con Date Creek, and Weaver mountains. It arises at tributed to regional erosion and do\vncutting of about 2350 m and flows freely and intermit streams (Cooke and Reeves 1976). Floods also tently through bedrock canyons interspersed can cause local extirpation of aquatic species in with deep alluvial basins to its confluence with areas where habitat fragmentation has reduced the Gila River at about 240 m. South ofWick their ability to recolonize disturbed areas (Col enburg in northwest Moricopa County, Arizona, lins et al. 1981). In general, however, because a shallow bedrock Jayercauses perennial surface desert stream ecosystems evolved with flooding, flow for about 8 km. The bedrock-confined per they are able to resist or rapidly recover after ennial reach is supplied with alluvial and basin flood events (Fisher and Minckley 1978, Fisher fill groundwater stored in a deep basin located et al. 1982, Reichenbacher 1984). around Wickenburg (Jenkins 1989a, 1989b). Few desert streams in the Southwest have The watershed above this point is about 1800 not been modified to some degree by human km', approximately one-third of which is com activities. The opportunityarises infrequentlyto posed of mountains vegetated by PinllS ponder study large floods in relatively unimpacted sys osa fOl"sts. The remainder is rolling hills and tems. In February and early March 1991, rain valleys vegetated by Interior chaparral and Son stOlms caused extensive £1ocxling in Arizona. oran desertscrub species. Three-day rainfall totals within the \vatershed of The study was conducted along a gaining the Hassayampa River were 7.1 em (VVicken section of the perennial river reach (base flows 3 l burg station) to 10.1 em (Prescott station), com increase from 0 to 0.11 m s- ) at an elevdtion of prising about 25% of the annual average rainfall. 600 m within The Nature ConselVaney's Has This resulted in peak stream flows of 368 m's-J sayampa River Preserve (Jenkins 1989a). The (>3000 times base flow level) at The Nature river has a gradient of 6 m km-I. The primary ConselVancy's Hassayampa River PreselVe, a channel has sandy bed sediments, a widthof1-,3 relatively unmodified riparian system for which m, and depth of about 0.3 m. The floodplain, there are pre-flood baseline data (Stromberg et which ranges from about 150 to 200 m in width, al. 1991). A continuing series of storms and in this paper is defined geomorphoJogically as that surface adjacent to the channel and built of spring snowmelt produced several smaller flood materials deposited in the present regime ofthe peaks through the middle of April. This event river (Graf 1988). This encompasses surfaces provided the opportunity to study the response vegetated by frasopis velutina that are up to 3 of the riparian ecosystem to a 10-y"ar return m above the water table (Fig. I), based on flood. Our primary objectives were to quantify evidence that substrate in such areas was flood (1) changes in floodplain topography resulting deposited (Burkham 1972, Minckleyand Clark from sediment deposition and scour; (2) survi 1984). The adjacent uplands slope down to the vcrship of dominant riparian trees (Populus fm floodplain with varying gradients. The climate is montii, Salix gooddingii, frasopis velutina, and arid, with average annual rainfall of29 em at the the exotic Tamanx pentandm) and shrubs (Bae Wickenburg station. eham salieifolia, Hymenoclea m01lygym, Tes The Hassayampa River PreselVe was histori saria sericea, and Zizyphus obt,tSifolia); (3) cally grazed and used ,,,creationally, but both post-flood seedling recruitment and '''getative impacts were eliminated in 1987 when the area reproduction of trees and shrubs; and (4) was acquired by The Nature ConseJVaney changes in cover and compositionofherbaceous (Richter 1992). The system may still be recov species. Secondary objectives were to compare ering from these prior impacts; however, there the effects of the lO-year return flood to those are no streams in the area that have been un of smaller prior-y"ar floods, including a 5-year grazed for long time periods with which tl,e 120 GREAT BASIN NATURAliST [Volume 53
Populus [rcmolllii I Salix Sooddingii Forest Willi ProsGpis Sllpling,s~ ---, I -
Prosopis veill/;IIG Woodland
WalerTablc ------'====.....:r:-_ Scale: 1__-11 = 5 III :-:-:-~ L.J Ab:l.lldollcd Channel fiymelloclO!a mflllncsra RivcrChanllei Shru l.J I:md
FiK. I. Ht'.pn~"entative Cf(~S section ofa portion ofthe 1JassayJ.mpa Hive!" floodplain. Tree 1)L.oj~hts ,md Jcplh to the water tahle iU-c to SCl.1Je.
l-lassayampa River ecosystem could be com rod per plot) throughout the floodplain. Depth pared. The herhaceous understOlY contains was measured in August 1990 (pre-flood) and many exotic plant species, but the overstOlY late March 199.1. and May 1991 (post-flood). species are predominantly native (e,g., P velu~ Fifteen ofthe rods muld not he relocated after tina, P. fremontii, and S, gooddingU) except for the flood. Relationships of sediment deposited ,l small component of 7.' pentandra. Portions of or scoured by the 1991 flood with floodplain the watershed are grazed by cattle ,Old urban elevation (i.e" height above the water table), ized, processes that may result in increased sedi ,Iistanee from the primaJy channel, and woody ment yield or increased peak llow velocities plant stem density were detennined with uni (Von Guerard 1989, Kondolf and KeUer 1991, variate nonlinear regression analysiS. Multivari Leopold 1991). at.e analysis wac; not utilized becausc variables were not independent (e.g., stem density and 1ETIIODS floodplain elevation). The relationship between avemge sedimentation within the floodplain Data were collected on stream discharge, and flow discharge was quantified with univari floodplain aggradation and degmdalion, woody ate regression, using data for the J991 flood and plW1t survivorship and recTilibnent, and herba for five smaller £1ood'i in prioryears (Stromberg CeOtL~ plant cover, spedes lichness, and Shan et al. 1991). non-Weiner species diversity. These data \..vere Tree and ShlUb Snrvivorship obtained during the 1991 flood year and dUling and Recruitment three prior years with smaller floods. Plant no menclature fdlows Lehr (1978). Stem density of woody plants was sampled within J00 permanent, nested plots distributed Flow Data amI Floodplain Sedimentation throughout the floodplain. Large trees (>10em Daily stream flow rate (m's-') during the stem diameter at a height of.l. m) were sampled study was measured automatically at a stream in 1989 and after the flood in 1991 in 10 X 4O-m gage located ca. 0.6 km downstream ofthe per plots. DensityofShlUbs, tree saplin~s(plants I yr posited or senured by the 1991 flood was old), and pole trees 0-10 em stem diameter at measured by sedimentation rods (i.e., stcel re a height of 1 m) was sampled in late March or infort:ement rods) at }OO pennancnt plots (one early April 1988, 1989. 1990, 1991, and again in 1993] RESPONSE TO 10-Yb:AR Rb:TUHN FLOOD 121
July or August 1991 in 2 X 2-m plots. Saplings, 'cC' J-- ~------l , shrubs, and trees were mapped in all years, ( allowing for more precise calculation of post IIJU I , flood revegetation ~md annual survivorship in ,,",• years with diHerent flood r~agnitudes. Tree and shnrb seedling densities were measured ,,' "(t :,u monthly in 1991 in4 X 4-dm plots to document ,, o post-flood seedling recruitment. (/1 SUlvivorship of shrubs, saplings, and pole n trees from 1990 to 1991 was analyzed in relation I to several environmental vmiables (stem den ,', I ____ ~ __ , _.L. sity, floodplain elevation, and distance from the () .'j , channel; and water depth, velocity, tractive \~Cr'jlll 01 - II ",','/,1 1\ '(11\1; shem- stress, stremn power, and sediment depos Fig, 2, Daily hydrograph ofthe IIassayampa HiveI' during ited dming the 1991 flood) with nonlinear re the 1990-91 water veal', Data for 12-26 Mareh were not gression cmaJysis. The flood flow parameters available. • were calculated from a calibrated HEC-2 flood plain model, and floodplain elevation and dis tance from the channel were determined from RESULTS cross-sectional surveys ofthe floodplain (Strom berg ct al. 1991). Survivorship was analyzed for Flow Data and Flooclphun Topography a composite data set of all shrub, sapling, and pole tree stems, andseparatelyfor three individ The 1991 flood had peak discharge of 368 m's-l on 1 Mareh (Fig. 2), a value about 3000 ual species (P frenwntii, S. goocldingii, and B. times greaterthan the base flow rate (0.1 m:1s 1), salicifolia). Survivorship \vas not analyzed for about 12 ti~es qrc~tcr than the 1.5-yr bankfull species with low mortality (e.g., P. velutina and dlscharge (30 n1' s ), and With a reCUITence in Z. obtusifolia) or those in fewer than 20 study terval of slightly less than 10 years (Jenkins plots (e.g., H. nwnogyra and T seneca). 1989a). Discharge remained above base flow values through mid-April. The flood inundated Herbaceous Cover nearlyall ofthe floodplain, in contrast toa5-year Cover of herbaceous vegetation was esti retUl11 flow that did not inundate high flood mated visually within 100 pennanent plots (1 X plains vegetated by P velutina (Table]). Peak 1 m) disbibuted among several different over flow velocity in the riparian zone in March 1991, storyvegetation types. Herbaceous cover in four as calculatedfrom the IlEC-2 floodplain model, overstory types (E. salicifolia stands, H. ranged from 1.7 + 0.6 m S-l in the near-stream flwnogyra stands, Populus-Salix forests, and P herbaceous vegetation type to 0.9 + 0.4 m S-I in velutina forests) was sampled in March and the high floodplain P velutina forests. For these same areas, peak water depth was 2.6 + 0.2 m September 1988, 1989, 1990, and 1991. Cover and 0..5 + 0.3 1Il (Table 1); and tractive shear in the streamside herbaceous type was sampled 2 stress was 12.7 + 7.3 kg 111- and 3.4 + .3.4 kg rn 2. monthly during these years to document rates Surface topography was altered during the ofpost-flood recovery. To compare within-year flood as a result of deposition of sediment and effects offlooding, herbaceous cover by species woody debris on floodplains, scouring of sedi was sampled in ten 1 X -m plots in flooded and J ment from channel hanks, and creation ofscour unflooded P. velutina forests in March, April, pools along the main channel and in overflow June, and July 1991. Depth of sediment was channels. The 10-year retUl11 flood deposited used as the indicator offlooding. Prnsnpis velu more than twice as much sediment (8 em) as a tina forests \-vere chosen for this analysis be' prior5-year return flood (3 em) (Fig. 3). Depo cause they occupied the highest floodplains and sition peaked (max-imum of47 em) onfloodplain encompassed areas with and without flood im sites that were 1-2 m above the water table and pact. Mean cover and species richness per plot declined in "hell curve" fashion on higher and were statistic"uly compared between flooded lower surfaces (Fig. 4). This pattern differed "md unflooded forests with Student's t test. from that for smaller prior-year floods in which 122 GREAT BASIN NATURALIST [Volume 53
TABLE 1. Water depth and flow velocityon the Hassayampa River floodplain during floods ofvarying recurrence intervals, by riparian vegetation type. Values are means ± standard deviation.
1 Water depth (m) Flow velocity (m 5- )
2-year 5-year IO-year 2-year 5-year IO-year Vegetation type flood flood flood flood flood flood
Streamside herbaceous 1.6 ± 0.2 2.4 ± 0.2 2.6 ± 0.2 5.2::!:: 2.0 6.0 ± 2.7 5.5::!:: 1.9 Populus-Salix saplings 1.1.::!:: 0.3 1.9 ± 0.3 2.1 ± 0.3 3.8 ± 2.3 5.2 ± 3.4 5.4 + 2.2 B(lc:dt(Lri~ ,m!id{(Jlia 0.8 ± 0.4 1.5 ± 0.5 1.7::!: 0.5 1.9 ± 0.6 2.7 ± 0.7 3.3 + 1.0 Populus-Salix pole trees 0.8 ± 0.4 1.6 :!: 0.5 1.8 ::!: 0.6 2.2 :!: 1.6 3.4 ::!: 2.4 3.6::!: 1.7 Hymenoclea. numogyra 0.3 ± 0.2 0.7 ± 0.4 0,9 ± 0.4 2.3 :!: 1.2 2.6 ::!: 0.3 3.2 + 1.2 Poplll1L~-S(llix forest 0.4 ::!: 0.4 0.8 ± 0.5 09 ± 0.6 2.4 ::!: 1.3 3.4 ::!: 1.3 3.5 ± 1.2 Prosopis oolutina forest 0.0 ± 0.0 0<0 + 0.0 0.5 ::!: 0.3 0.0 ::!: 0.0 0.0 ::!: 0.0 3.0::!: 1.4
1(j .-, ------r------,-- -.- 1 .,0 . I - 2 ~) - •I ~ '-"/ H , ,-, .-" '0 ,,'0 - " ~ I j '" ,• u, " ," W / 0 Q ,,' 'n > G , ",,' , ~ ,n / 0 ._----- I.,] " - , "w,n --10
- 1 ~) o·-_~, 5-1 1-1.:;' 1.5-2 2-2.~ 2~,-,~ 3-3,~,
[)ISCIII\R(;L (rnJ!s) FLOODPL.AIN eL[VATION (rn)
Fig. 3. Sediment deposition in reilltion to flood mllgni Fig. 4. Sediment deposition (means and standard devia tude of the I-lassayampa River. Values represent average tion bars) on floodplains ofvarious heights above the water sediment deposition in the floodplain during the six largest table during a lO-year return flood in 1991 in the Has floods in the period 1987-91. The regression equation is: y sayampa River. ~ OlXl8 + 0.022,," ~ .97; df ~ 5; P <01.
tion. Vegetation types with abundant woody sediment decreased cUlvilinearly with increas stem density (e.g., B. salicifolia and E fremon ing floodplain elevation (Stromberget al. 1991). lii-S. gooddingii pole stands) accumulated Many low floodplain surfaces «1 m high) adja more sediment than did types with lower stem cent to the main channel had a net loss of sedi density (e.g., H. nwrwgyra stands) (Table 2). ment in 1991, although these areas regained some sediment during small flood surges in Tree and Shrub Survivorship April. The channel bed became wider and sId lowerinmany areas buthaddeepenedand again Woody plants growing on high floodplains where flood impacts were least had highest sur become entrenched in areas by midsummer of vivorship of the 1991 flood. For the composite 1991. sample of shrubs, saplings, and pole trees, sur Sediment deposited during the 1991 flood vivorship increased significantly as functions of . was related to density of woody vegetation and flood water depth (Fig. 5), floodplain elevation, to topographic position inthe floodplain. Flood and distance from the primary channel (Table plain elevation and woody stem density were 4). Mature trees ofP velutina, E fremontii, and negatively correlated (? = .25, P < .01, df = 84), S. gooddingii grew on floodplains higher than 2 and deposition was related to hoth factors: ? = m above the water table and had 100% survivor .11, P < .02, df = 84, positive relationship for ship (Tahle 3). Saplings of P velutina grew pri woody stem density; and? = .15, P < .01, df = marily in the understory of P fremontii-S. 84, negative relationship for floodplain eleva- gooddingii stands andalso had high survivorship 1993] RESPONSJ; TO 10-YEAR RETUHN FLOOD 123
TABIF- 2. Depth of sediment deposited or scoured on the Hassnyampa River floudplain during floods of varying recurren<:-'C interval.., by vegt:tatioll type. Mean floodplain height above the water table, rlistance from the strcotm chanTll;:l. and density of ~ stems are iTlclicated [or each vt::;getaUon tyIlt:. Values are means::!: standard deviation. ---~------Height above Distunre Woody Sediment (em) water rrvm stem table channel densil~ 2-year 5-year 10-ye Strc.~rnside herbaceous 0.4:t 0.1 4=4 2,3:!: 2.7 !-J.8:': 2.7 12.8 ± 4.7 -.1.l.2:!:11.O Populus-Salix s~)lings 0.7 :t 0,3 8:t8 9.9 ~ 125 4.3 ·1- 4.3 5.4 ~ 3.0 10.0 ::!: 6.2 Baccful~'saUciuha 1.0 :!. 0,.'5 22:t 16 4,!j i: 4.9 2.7 ::!: 3,0 5.1:I:t 5.7 13.4. ::!: 12.2 Populus-Salix poles 1..3 :! 0.5 22 ± 21. fl.9 :!: 4.6 4.2 :!: 4.9 5. I :!: 4.2 14.7 ± 12.5 Hymenor1cA numogyra 2.070.5 3,5 =. 9 2.0 ~ 2.1 0.9 :I: 1.8 0.9 ± 1.J S.k ± 5.4 POIJulus-SaJix forest 2.2!" 0,7 48 + 23 0.3' 0.8 0.3,1..3 1.7±2.6 7.R :!.: 8.2 ProS - .- ._- (82%). Pole trees of S. gooddingii, P. fremontii, "'0, I and T peTiWndra grew on mid-height flood plains 1-2 m above the water table and had BO respective survivorship 01'93%, 73%, and 38%. Tamarix peTiWndra was the only one of these n 60 I three species that had much lower survivorship n", ofpole trees in 1991 than in prioryears. Saplings ~ n> ofthese three species grew on floodplains < I m . , if' above the water table, and each had about 35% 20 SUrvivorship ofthe 1991 flood. Smvivorship of the 1991 flood by poles and saplings of P. fremontii was siguiflcantly related to floodplain elevation, distance from the rLOOo WAfeR DCPIII (rn) stream, and depth ofll,x,dwater (Table 4). Salix Fig. 5. SUrvivorship of riparian shmh'i, snplings, and gooddmgii survivorship showed the same small trees (poles) ill the Hassayampa Hivcr Ooodplain. trends, bllt relationships were not significant. 1990--91.. in relatiun to flood water depth dassl.'S. Popuuu; fremontii poles on lloodplains 1-2 m above the water table had 94% + 10 surviV'.l1, dingii saplings had greater survivorship than P. compared to 60% + 40 for those on floodplains fremontii saplings. 80% >1.5 m deep (Fig. 6). Sediment deposition, for H. nJ,onogyra, a species that grew on flood shearstress, stream'power, and velocitywere not plains averaging about 2 III above the water signiflcantly related to sUrvivorship for either table; and <20% for T sericca, a low-floodplain species. Betweenyears, annual survivorship for species (1.3 + 0.2 m) that sustained much stem P fremontii and S. gooddingii saplings de breakage. Stem SUrvivorship averaged 50% for creased significantly as annual maximum flood B. salicifolia, the most abunmmt shmb in the magnitude increased, with, for example, 30% of floodplain. This species formed dense stanels P. frerrwntii saplings surviving the 1991 flood, primarily on low fl TABU': 3, Annual survivorship of dominant riparian trees and shrubs in the Hassayampa River floodplain, Data are for years with a 5-year return flood (1988-89), Size class Survivorship (%) or growth Species form 1988-89 1989-90 1990-91 , h Pro.\'opis veluHoa mature tree' NS NS 100 Salix gooddingii mature tree NS NS 100 Po/minI' frenwntii mature tree NS NS 100 Pro,w)'pis veultiJUl pole tree 100 91 100 Salix gooddingii pole tree 87 80 93 PoJ!lllus frclIwntii pole tree 88 89 73 '[amarix pcntandra pole tree 95 87 38 Prosopis vellltina sapling 76 87 82 Salix gooddingii sapling 64 78 36 Populus frclIumtii sapling 43 58 30 Tar/wrix pcntaodra sapling 84 75 37 ZbW'l1ls ohtlls!fo!ia shrub 100 100 100 Hymenoclca mOl1of!,yra shrub 96 100 83 13acdwris ,)'alicifiJlia shrub 100 100 51 Tessaria sericea shrub 100 100 17 'MatillT h~~'s hIlW ~t(,nlS >.l (J em dianwtC"'; pdC' trcC's lHlvc ~tel)'\,1 I-10 em diameter: sapli ng~ have stems <1 ~l)l dinmet~r "nd ~re ~r~l\t",r than 1. year in age. ":-.IS = not sampl"d, TABU'; 4. Hegression coefficients (? values) relating dessication. By the end ofsummer there were 5 three physical parameters to survivorship ofa lO-year return seedlings m-2 on floodplains <1 m above the no(xl in the Hassayampa River, by saplings and pole trees of POllUlusfrenwntii and Salix gooddingii; and by a composite water table (Table 5), a value suffiCiently high to group of riparian shrubs, saplings, and pole trees. eventuallyproduce a mature forest with chm " ------ • ';1\1 IX- C;ClOrmINCIi 1 • If\lMO~rll • (> i-'Oi-'L!Il!S 80 • -- • le, .------_ " . ------" ------o (;0 - u • , , ()------", ~()- ~ ",Q ------o • o > n > > • ~ "-, ::..;0 Q SAliX S/IPIIN" • '" '" 20 • SALIX POLE " POPULUS SN'I.IN{; • PUf-'UUh POLE 0 0 .. , 0 -----, ------, , T------,----" " o 50 100 1~)O )00 ~~;O"O() _',~)O 4()(1 00 0 1.0 , ;I ;I _~; :,,0 J.S o " " " I LOOD WAI1_1~ ULPIII lrnJ IW;(;III\RGl (rn3/s) Fig. 6. Survivorship ofsapling:'> and small trees (poles) of Fig. 7. Annual survivorship of saplings of Populus fre Populusfrenwntii and Salix gooddingii, 1990-91, in relation rrumtii and Salix gooddingii along the I-Iassayampa River to maximum water depth during a lO-ycar return flood. floodplain in relation to maximnm annual flood flow rate. 2 Regression equations are: y = 59 - O.08x, r = .99, df = 2, 2 P < .01 (P.fremnntii); and y = 82 - O.12x, r = .97, df = 2. 400 ------P < .05 (S. gooddingii). C.JMAf,CII ~~!lY " !QMlJULY ,l rrwnspelie~is). _OCTOH,R Cover in these areas increased ,o ncarly to pre-flood levels by September, Cover ,> 300 I within higher-elevation vegetation types (e.g., ",z Populus-Salix forests) remained low as of late '" summer. Within P. velutina forests, areas that " '"0 C"z were flooded had lower cover but greater rich , n ness and diversity of species thronghont the " , ",'0 summer compared to areas that were not ~ ~ flooded (Table 7), Unfloodcd and flooded areas 0 I. • p,j", -~ " n 0-- .') .,,-- 1 I -- 1 ,,) , , 7.5--:, j-.'.:i S ? " in the P. velutina forest were both initially domi I-I OUI)F'I i\IN HFICI-IT (m) nated by two exoticwinter-gemlinating annuals, Honleurn lepor'inu.m and Sisymhnum irio. Fig. 8. Density of PO]Julusfrerrwntii in relation to water These two species continued to dominate un table depth, by month during 1981. flooded areas througbout spring and early sum mer, Flooded areas, in contrast, had about 1/6th the cover of nuflooded areas, and about 4-5 Herbaceous Cover 2 times as many species (e.g., 9.2 + 1.9 m- vs. 1.9 z Spling herbaceous cover in all vegetation + 0,5 m- , April data), These included several types except that of the highest floodplains (P. native anIlual forbs (e.g., Amaranthus palmeri, velutina forests) was less abundant in 1991 than Bowlesia incana, Amsinckia interm.edia, Cilia si,n11.ata, Loll1.S humistratus, Microseris lineari in prior years (Table 6). Herbaceous cover un folia, Xanthium stntrnari11.rn, and Verbesina der P. frerrwntii-S. gooddingii forests, for exam encelioides) and several exotic annual Forbs and pIe, averaged 8% in 1991 compared to 25-43% grasses (e.g., Brom.us Tltbens, Herniaria cinema, in plior years. Herbaceous cover on stream Solanum rostyaturn, and Trih11.lus terrestris). hanks 'md in B, salicifolia stands was 16% and 11% respectively in late March 1991 compared to 38% and 34% in the prioryear. Coverin these DISCUSSION two areas was composed primarily of rhizoma tous grasses (the native Pl1spalumd;",tichum and Riparian systems are noted for their resil the exotic Cyrwdon Mctylon) and also cun iency, i.e" the ability to quickly rctum to pre-dis tained lesser amounts ofother natives (e.g., 1!/ turbance conditions. Rapid grmvth rates, high phl1 domingensis and species of funcus) and fecundity; and capacity for asexual reproduction exotics (e,g" Melilotus albus and Polypogon are among the factors that allow rapid recovery 126 CHEAT BASIN NATUHALIST [VolumeS3 2 TAil] ~.; S. Average seedling densities (no. m-- ) during the period of maximum abundance and at the end of the growing season for three riparian tree species on IIassayampa River floodplain surfaces PorJU!US fi-emontii Salix gooddingii Tamar-ix pentandm April October May October June Odoher 1988 111 0 385 {) II 0 1989 12 0 2 {) <1 0 1!J.-)O 0 0 <1 0 0 () Hll-) 1 205 5 618 3 5 0 TABLE G. IIerlmccolls cover (%) along the I-Iassayampa Hiver floodplain, hy vegetation type, from 1988 to 1991. Values arc means:!:: standard deviation. ------1988 1989 1990 1991 Streamsi Bacclwri..s sa[ici!o[ia shrubland March 22 ± 18 19 ::':: 19 34::':: 26 11 ± 24" Sept. 38::'::: 36 41 ± 35 2'5 + 21 41 -4- 4.1 HYlIWlwclea nW1).o{!,ym shrubland March 19 ::'::: 1R 20 ~ 24 24 ± 21 4 ± 4!' Sept. 7, [j 15:t: 13 12::'::: 11 13::'::: 13 Pormlus-Sa!ix f<)rest March 43::'::: 27 2.'5 + 2] 34 ± 31 Ii ± 12<1 Sept. 18 + 29 21 ± 28 21 ::'::: 2.'3 5::':::6 Pros0J!is Oe!UtifUi forest March 84 ± 17 3,'5 ± 21 58 ± 26 52 ± 45" Sept. 7 --.! 12 12 ::'::: 16 1O± 17 6::'::: 13 -- TABll'; 7. CANer, richness, and Shannon-Weiner diversity of herbaceous understory species in Prosopis vehLtina forests that were and were not inundated during a 1O-year return flood in March 1991. Values for mver and richness are means + standard deviations. Species Cover ('Yo) Species richness diversity Month Flo March 7±7 71 :!: 30" 2.3 ± 0.5 1.9 + 0.6 0.53 0.9J April 18:+:4 79 ± ,'36" 9.2 ± 1.9 1.9 ± O,S" 2.46 0.73 IlllW 12 ± 6 84 ± 24° 10.1 ± 3.1 2.2 ± 0.8 0 2.53 0.45 Jll1y 16 ± 9 88::'::: 11" 7.9 ::'::: 3.0 2.6 ::'::: 2.3" 2.31 1.23 'Siplillnlll( r!i/Tefl'lln' al I' < .o.'i. of riparian plants after disturbance (Stromberg The lO-year retum flood in the Hassayampa and Patten 1989, Cecy and Wilson 1990). River inundated most of the floodplain and de Densely vegetated floodplain cc'osystems also pOSited a net average ofB cm ofsediment (maxi can be resist:mt to floods, in the sense that floods mum of 0..5 m). Low-elevation floodplain pass over them without scouring vegetation or surfaces had greatest flow velocities (to 7 m S-l) substrate (Hendrickson and Minckley 1984). and water depths (to 2.8 m). The native riparian 1993J RESPONSE TO lO-YEAK RETURN FLOOD 127 vegetation showed a mixture of resistance and flood) (Stromberg et at 1991). This present resilience to this Hood disturbance. Species on studyconfirms the role oflarge floods inincre,-l'i high floodplains (e.g., P velutina and Z. obtusi ing age-class diversity f(Jr these episodically re folia) had no mortality, while those onlower-ele cruiting species. vation floodplains variously had mortality The exotic T. pentandm co-occurred with followed by seedling recruitment (P frenwnUi Populus and Salix but had greater mortality of andS. gooddingii) orbyvegetative reproduction pole trees than did the native trees. Mortality of (e.g., Baccharis salicifolia). T. pentandra more likely resulted from intoler Prosopis velutina was the dominant tree on ance to physical flood effects than £i'om physi high floodplains (ca. ,3 m above the water table) ological intolerance to inundation (Warren and and had high survivorship oftrees and saplings. Turner 197,5, Irvine and West 1979). Tmrwrix It did not show post-flood seedling recruitment, pentandra had low post-flood seedling estab consistent with plior studies indicating that P lishment, due in part to a low density ofmature velutina seeds genninate primarily after late seed-prodUcing trees in the IIassayampa flood summer floods (Stromberg et a1. 1991). Populus plain and in part to the fact that the £I(){xl oc frenwntii and S. gooddingii trees grew on flood curred several months prior to 1: pentandra plains 2-3 m high and also had high survivor seed gemlination and thus did not moisten po ship. Young trees and saplings of these two tential germination sites at an appropriate time species were on younger, less aggraded flood (June through October). Additionally, mucb of plains and sustained some mortality. Salix good the available "gennination space" during its ger dingii saplings and poles bad lower mortality mination period was preempted by herbaceous than did Populus fremontii, perhaps because of cover and by seedlings of P frenwntii and S. greater stem pliability and tolerance to satura gooddingii, species that precede Tarnarix pen tion (McBlide and StnJlan 1984, Hunter et a!' tandra in the chronosequence of tree species 1987). Survivorship ofboth species was greater gennination at the IIassayampa River. on sites where flood waters were shallowest, a Vegetative reproduction is a common post factor reported to he an important determinant disturhmce revegetation mechanism in flood of flood survivorship in other Jiparian systems plain systems (Gecy and Wilson 1990) and was (Stevens and Waring 1988), Thc rclationship demonstrated hy all shruh species in thc Has between water depth and survivorship may be sayampa River floodplain that had flood mortal an expression ofeffects offlood hydranlic forcc ity. Extent offlood mortality ofshmb species at on plant removal or mortality via abrasion and the Hassayampa varied with their topographic stem breakage, rather than of a causal relation position in the floodplain. ZizyphllS ohtlls!folia, ship between root saturation and mortality. Al a species ofhigh floodplains (ca. 3 m above the though correlations of mortality with flood water table), had no mortality. Baccharis salici velocity and shear stress were not statistically je)lia underwent a .50% decline in stem density significant, this may have been due to chaotic during the flood but increased to pre-floodden movement ofwaterand sediments on the flood sities by late summer pIimarily via stem sprout plain, which arc not adequately represented by ing. Hymenoclea nwnogyra and T. serieea are flood-simulation models such as HEC-2. both clonal shrubs that spread via root sprouts The 1991 flood created optimal seedling re after mechanical injul)' (Gal)' 1963) and via cruitment conditions for Populus frenumtii and shoot sprouts after stem bmial. [-Jymenoclea Salix gooddingii by scouring channel banks and nwnogyra compensated for flood mortality by depos-iting new sediment on stream banks, re vegetative reproduction; but this was not the ducing herbaceous and ovcrstOl)' competition case for Tcssaria seneca, a low-floodplain spe (at least temporarily), and moistening flood cies that had high flood mortality. Other studies plains at an appropliate time (during seed dis also have reported low flood survivorship for persal) and place (moderately high surfaces Te.r;;saria serieca (Stevens and Waring 1988). above the zone offrequent summerflood scour) Vegetative reproduction also was the domi (Stromberg et at 1991). Tree-ring studies have nant revegetation methodfor herbaceous plants shown that-Pfrcmontii and S. gooddingii estab along stream banks and low-elevation fItx)(l lish in large scale about once a decade \~~thinthe plaiIls. Cover in these areas declined hy ahout Hassayampa River system, dming or after years half after the flood but recovered to pre-flood with large flows (>250 m:\l; 7-year retunl levels by late summer. Flood-tolerant perennial 128 GREAT BASIN NAnJHALIST [Volume 53 rhizomatous gmsses (P. distichwn and C. dacty established in 1959, 19.52, and earlier (Strom ton) dominated these area.s during and prior to berg et 'II. 1991). The] a-year return flood prob the flood, but species less tolerant of high flow ably reached a "geomorphic threshold," that velocities (e.g., TY1Jha clontingensis) may ulti being the level at which suhstantial change in mately increase in abundance during flood floouplain mOlphology and vegetation begins to intelim peliods (Fisher et a1. ] 982, IIen occur, based on studies of other desert rivets driekson and Minckley 1984). Understories of that implicate the 5-year return flow as a thresh high-elevation floodplains showed changes in old discharge for channel and floodplain insta cover and composition after the 1991 flood. bility (Graf 1983). Prior to the flood, P. velutina forests were domi Other potential effects ofthe flood on lipar nated by dense, nearly monotypic stands of ex ian vegetation such as changes in plant produc otic annual species (e.g., Hordeum rnurinum) tivityas a result ofnubient orwater pulses were that probably had become established dming not addressed in this study, nor was the role of past years ofcattle grazing and other exogenous vegetation in moderatingflood processes explic disturhanees (Wolden et a1. 1991). After the itlyaddressed. flood these areas had lower cover hut greater Data in this paper suggest that floodplain lichness ofherbaceous species and greater rela vegetation aided in stream hank stabilization tive abundance of native annuals. We speculate and sediment trapping, importaIlt functions of thatcompositional changes were due to reduced wetland and liparian vegetation (Fisher and competition \vith entrenched exotics, an influx Minckley 1978, Cooper et ill. 1987, Sullivan and of flood-home seeds from upstream areas or Stromberg 1992). The vegetation also may have other vegetation types \vithin the floodplain, or enhanced groundwater recharge and reduced altered edaphic conditions resulting from depo the dmvl1stream impact offlood flows by reuuc sition of sediment \vith diHerent texture or nu ing flow velocities and increasing water reten llient content (Stevens and Waling 1988). tion time within the floodplain (Burkham 1976, Fluvial processes including floodplain aggra Beschta and Platts 1986). dation and formation of microrelief patterns (e.g., backwater depressions) contlibute to the ACKNOWLEDGMENTS diversity and "mosaicism" ofripmian phmt com munities in many flood-driven ecosystems (Kal \Ve thank The Nature Conservancv, for al~ liola and Puhakka 1988). Within the lowing the use of the Hassayampa River Pre Hassayampa floodplain, as well, variable sedi serve as a research site. Review comments by ment deposition and scour patterns contributed \'V. Calier Johnson and anonymous reviewers to "patchiness" within the ripaIian floodplain. are greatly appreciated. For example, localized light gaps were formed in area.<; with major debris deposition, and scour LITERATURE CITED pools (i.e., backwaterdepressions) were fonned along main channels and in overflow channels. BELL, D. T. 1974. Tree stratum composition and distribu Floodplain sedimentation accentuated the ex tion in the streamside f(wpst. American Naturalist 92: istence ofhydrological gradients (e.g., gradients ,1,5-46. of depth to groundwater), which contribute to BESCIITA, R. 1.., /\]\]) \V. S, PLATTS, 1986. :\iIorphological signilkance ofsmall streams: significance amI function. floristic diversity \vithin riparian ecosystems of \'Vater Hesources Bulletin 22; :*19-379. the Hassayampa River and elsewhere (IIupp BHAV..... H.D, J., C. AMAJ\OS, Al\D G.I'AUTOU 1986. Impact of and Ostereamp 1985, Bravard et aI. 1986, civil engineering works on the successions of t:ommu Stromberg et 1991). nities in a fluvial system. Oikos 47: 92-11l. 'II. BUI1.KIIAM. D. E. 1972. Clmnnelchanges in the Gila River The IO-year retunl flood resulted in greater in Safford Valley, Arizona, 1846---\970. United States floodplain aggradation and greater woody plant Geological Survey Professional Papcr 65.5G: 1-24. mortality than smallerprior-year floods. Effects ____. 1976. Hydranlk effects of changcs in bottomland vegetation on three major floods, Gila River in south offloods larger than the lO-year event can only western Arizona. U.S. Geological Survey ProfeSSional be speculated upon. A lOa-year retnm flood in Paper 65SJ: 1-14. 1970 in the IIassayampa River (Jenkins 1989'1) COLLINS, J. P, C. YOUNG, JB .. J. HmVEI.!.. AN"n W. L. apparently did not cause extensive loss ofripar :\tIl !\CKLEY. ] 981. Impact of flooding in a Sonoran Desert stream including elimination ofan end,mgered ian vegetation, as indicated by the presence of flsh population (Peociliopsis o. occidentalis Plx"cilii extensive stands ofPopulus and Salix trees that clae). Southwestern Naturalist 26; 415-42.3. 1993] RESPONSE TO lO-YEAH RETUHN FLOOD 129 COOKE, R. U., AI\'I) R. W. REEVES, 1876. Arroyos and envi proceedings on heauwaters hydrology. Amcrkan ronmental change in the American Southwest. Claren Water HesOllrces Association, Bethesda, Mmyland. don Press, Oxford. JOllI\SON. \\". c., R. L BUHCESS, A:'<]) W. R. KEi\MMEHEH. OJOPF:H, R, 1. W. GILLIAM, R. B. DA"'JF:LS, 1\\1)) w: P. 1876, Forest overstory vegetation and environment on HOBAHGK 1987. Riparian areas as filters [or agricul the Missouri River f](xldplain in North Dakota. Eco tural sediments. Soil Science Society of America Jour logical Monographs 46: ,'59-S4. nalSl: 416-420. JU:' SPA ilKS. H. E., P. B. BAYLEY, S. 1. KOJJLEH, AND 1. L. Os' the Society for vVetland Scientists 13th annual meeting, BOliN I':. 1990. Disturbanc'C and recovery oflarge 1100d In press. plain rivers. Environmental Management 14: 699-709. VON GUEHAHD, P. 1989. Effects of land use on sediment STI\vr':NS. L. E., AN)) C. L. WAlliNG. 1988. Effects of'post yield, southeastern Colorado. Pages 223--241 in W. W. dam flooding on riparian substrates, vegetation, and Woessner and D. E Potts, eds., Symposium proceed invertebrate populations in the Colorado River corri ings on headwaters hydrology. American vVater Re dor in Grand Canyon. USD! Bureau of Reclamatiun sources Association, Bethesda, Maryland. Glen Canyon Environmental Studies Report 19. WAHHEN, D, K., AND R. M. TUI\Nfi.H, 1975. Salt cedar STJ\O~'lBEllC, J, C., AND D. T. PATTEN. 1989. Early remvery (Tamarix chinensis) seed production, seedling estab of an eastern Sierra riparian system following forty lishment and response to inundation. Journal of the years of stream diversion. U5DS Forest Service Gen Arizona Academy ofSdence 10; 135--144. eral Technical Report PSW-llO: 399-404. \VOLDEN, L., J. STHOMBEHC, D. PArrEN, AND H. RICliTEH, ____"1992. Mortality and age ufblack cottonwood stumls 1991, UndcrstOlY restoration in three riparian forest along diverted and uIHhverted streams in the eastern types (Arizona). Restoration amI Management Notes Sierra Nevada, Califi)fnia. Madrniio 39: 205-22:3. 8,116-117. STllOMBI':HG, J. G, D. T. PATTEN, AND B. D. HrCIITEH, 19\;)1. Flood flows alld dYllamic.~ of Sononm riparian Received 1 November 1991 forests. Hivers 2: 221-235. Accepted 1 Deccmher 1992 SULLIVAN, M, K, AND J. G STHOMBEHG, W\;)2. Wetland fnnctions in Southwest riparian forests, Proceedings of