GEOMORPHIC AND RIPARIAN INFLUENCES ON THE DISTRIBUTIONAND ABUNDANCE OF SALMONIDS IN A CASCADE MOUNTAIN STREAM'Kelly M. S. Moore and Stan V. Gregory2

Abstract: Abundance of resident cutthroat (Salmo clarki)and rainbow (Salmo gairdneri) trout was generally 1.5 to3.5 times greater in unconstrained reaches than in con-strained reaches of Lookout Creek, a fourth-order trib-utary to the McKenzie River, Oregon. The presenceof adult rainbow trout depressed juvenile abundance inpools with little habitat complexity but had no effect inpools with more heterogeneous structure. The greaterabundance of trout in unconstrained reaches was relatedto habitat structure, the influence of the riparian canopyon stream productivity, and the effect of channel mor-phology on stream hydraulics. Valley floor landforms aremajor determinants of channel complexity and habitatstructure, providing a hierarchical geomorphic contextfor interpreting riparian influences on patterns of abun-dance and distribution of salmonids within a basin.

The viability of cutthroat trout (Salmo clarki) popu-lations in steep Cascade Mountain streams is dependenton the recruitment of juveniles within a stream reachand the quality of rearing and refuge habitat. Recruit-ment is particularly important because cutthroat troutin these streams are residents that may complete theirlife history within a 20- to 100-m reach (Miller 1957; Wy-att 1959; Aho 1977). Factors that influence recruitmentinclude availability of suitable habitat and the presenceof predators. Age 0 trout initially occupy the marginsof the stream channel and move to faster-deeper habi-tats as they grow. Habitats at the lateral boundary ofthe main channel are structured by the interaction ofstreamflow with boulders, wood debris, and the geomor-phic constraints of the valley floor.

Valley floor structure can influence stream ecosys-tems by influencing channel morphology and by regu-lating energy input and processing (Gregory and others1989). Mechanisms of this influence become apparentwhen drainage networks are organized hierarchically byreach type, channel unit, and habitat subunit. Reachtypes are delineated by the type and degree of localconstraint imposed by the valley wall at the channelmargin. Reaches are constrained by bedrock intrusions,landslides, earthflows, and alluvial fans. Streams withinconstrained reaches tend to be relatively straight, single-channels with little lateral heterogeneity. Unconstrained

reach types are characterized by complex, often braidedchannels with extensive floodplains.

The stream channel in both constrained and uncon-strained reaches is composed of longitudinal sequencesof channel units with distinct hydraulic and geomorphicstructure (Grant 1986) that are longer than one channelwidth and identifiable as pools, riffles, rapids and cas-cades. Channel units are divisible into habitat subunitsless than one channel width in length. The subunit scaleof hydraulic and geomorphic features corresponds to de-scriptions of fish habitat frequently used in ecologicalresearch. Subunits at the stream margin (lateral habi-tats such as eddies and backwaters) are characterizedby low velocity, heterogeneous substratum, abundantdetritus, and structural protection from high discharge.This combination of physical and biotic conditions pro-vides gradients of depth and velocity, cover, and accessto invertebrate food that make lateral habitats partic-ularly suited to the requirements of young-of-the-yearcutthroat trout (Moore 1987). The importance of off-channel pools, side channels, and tributaries for bothrearing and winter habitat has been well documented(Bustard and Narver 1975; Tschaplinski and Hartman1983; Hartman and Brown 1987). These studies havefocused on the importance to juvenile salmonids of habi-tats adjacent to the main channel. However, the effectsof habitat complexity, in the context of valley floor geo-morphology, has not been examined.

In an earlier study of riparian influence on cutthroattrout populations, Moore and Gregory (1988a) observedthat the abundance of age 0+ fish was generally pro-portional to the area of lateral habitat in third-order,Cascade Mountain streams. In a manipulation of sub-unit structure, Moore and Gregory (1988b) found thatincreasing lateral habitat from 12 percent to 24 percentof total stream area resulted in 2.2 times greater den-sity of age 0+ cutthroat trout. Juvenile trout popula-tions were virtually eliminated in stream sections wherelateral habitat was reduced. In the present study, theobjective was to examine the relationship between ju-venile trout and lateral habitat subunits at the streamreach and channel unit levels of organization. Also, theabundance of adult trout relative to reach type was con-sidered as were possible effects of physical structure oninteractions between juvenile and adult fish.

I Presented at the California Riparian Systems Conference; September 22-24, 1988; Davis, California.

2 Graduate Research Assistant and Associate Professor of Fisheries, respectively, Department of Fisheries and Wildlife, Oregon StateUniversity, Corvallis, OR, 97331, USA.

256 USDA Forest Service Gen. Tech. Rep. PSW-110. 1989.

125(r)E. co

0 75q-C

o 50cc

Methods

Observations of fish populations and measurements ofstream structure were made in Lookout Creek, a fourth-order stream in the H.J. Andrews Experimental Forestin the Cascade Mountains, Oregon, USA. In constrainedreaches, this stream flows through 450-yr-old stands ofconifers dominated by Douglas-fir (Pseudotsuga men-ziesii) and western hemlock (Tsuga heteropkylla). Inunconstrained reaches, conifers are less abundant andnear-stream vegetation is dominated by willow (Salixsp.) and red alder (Alnus rubra) Stream reaches weremapped and characterized in terms of a constriction ra-tio. Constriction ratio equals the width of valley floorless than 3 meters above low stream flow divided by theactive channel width. At Lookout Creek, the least con-strained reach (constriction ratio = 5.6) is located ina depositional area above an earthflow-constricted reachand has a wide valley floor and multiple secondary chan-nels. The most constrained reach (constriction ratio =1.3) has a single channel flowing through a steep walledcanyon at the toe of an earthflow. Mapping evaluatedthe degree of constraint on the stream and also measuredgeomorphic channel units and habitat subunits for 6.2km of Lookout Creek. Habitat complexity in channelunits was defined arbitrarily, based on the distributionof subunits. If one subunit class comprised more than70 percent of the channel unit area, the unit was con-sidered to have low complexity. If the channel unit hadhigh subunit richness and evenness (four or more subunitclasses, each comprising at least 15 percent of the area),the unit was considered to have high complexity. Dif-ferences in channel unit and subunit habitat area wasexamined with a Kruskal Wallis test of ranked classes(Sokal and Rohlf 1981).

Fish populations in Lookout Creek were censused bysnorkeling observation during the summer of 1987. Allreaches, channel units, and habitat subunits were exam-ined. Divers recorded the species, length, and habitatuse of every fish observed. Cutthroat and rainbow trout,Sculpin (Coitus spp.) and dace (Rhinichthys sp.) arethe only fish present in the study reaches. The distribu-tion of habitat sub-units within each reach was measuredduring summer base flow in 1987 and during winter baseflow in 1988.

weeks. The abundance of juvenile cutthroat trout .Lookout Creek was greater in unconstrained and semi-constrained reaches than in constrained reaches. Abun-dance also increased along a downstream to upstreamgradient, but, except for the extreme downstream reach,the density of juveniles was always greater in less con-strained reaches than in adjacent reaches with compar-atively greater constraint (fig. 1).

Age 1+ and older fish had a distribution that wassimilar to that of juveniles, but the pattern was modi-fied by the presence of large structural elements (boul-ders and wood) in the most constrained reach (fig. 2).The greatest density of fish was generally in the uncon-strained reaches. However, one constrained reach had aparticularly high abundance of adult trout. This reachwas at the toe of an active earthflow that has intro-duced numerous large boulders (> 2 m diameter) tothe channel. These boulders created channel roughnessthat entrained large woody debris and modified streamstructure to create a variety of habitat types within the

reach. Deep pools and backwater habitats were partic-

ularly abundant in this reach.

'50

= UNCONSTRAINED

ES SEMI—CONSTRAINED

CONSTRAINED

25

0

Results DOWNSTREAM

PSTREAM

The pattern of habitat utilization was the same ineach of the reaches of Lookout Creek. After emer-gence, juveniles established territories in lateral habi-tats exclusively and remained there for at least six

USDA Forest Service Gen. Tech. Rep. PSW-110. 1989.

Figure 1 — Abundance of juvenile cutthroat trout insequential reach types of Lookout Creek during thesummer of 1987. Reach types are classified by the degree

of constraint on the active stream channel. Abundanceexpressed as number of fish per 100 m of stream length.

257

3 100

crt80

OE0 60

I-D 400

20.zLI 0

NM LARGE ADULTS PRESENT

CI LARGE ADULTS ABSENT

LOW HABITATCOMPLEXITY

HIGH HABITATCOMPLEXITY

Figure 3 - Density of adult and juvenile cutthroat troutin channel units of Lookout Creek. Density expressed as

number of fish per 100m2 of wetted channel area.

150

125

100

75

•

50

25-

0DOWNSTREAM UPSTREAM

Figure 2 - Abundance of adult cutthroat trout insequential reaches of Lookout Creek during the summerof 1987.

0 UNCONSTRAINED

C23 SEMI—CONSTRAINED

MI CONSTRAINED

IN

The pattern of fish abundance in the study reacheswas influenced by the distribution of channel units withineach reach and the differential utilization of channelunits by both adults and juveniles. The density of adultcutthroat and rainbow trout was greatest in pool chan-nel units (fig. 3). Juvenile cutthroat trout were foundin all channel unit types, but had the greatest densi-ties in riffle and secondary channels. The differencesin juvenile abundance between constrained and uncon-strained reaches was not attributable to simple differ-ences in channel unit structure between reaches. Thearea of secondary channels was greater in unconstrainedreaches than in constrained reaches (9.0 percent and 2.3percent respectively. P < 0.05). There was no differencein the area of riffle channel units in different reach types(P > 0.10). The average proportion of total stream areain pool channel units was greater in constrained reaches(37.7 percent) than in unconstrained reaches (21.7 per-cent).

Unconstrained reaches had more complex channelunit and subunit structure than did constrained reaches.Channel units were shorter and the sequence of channelunit classes were more varied in unconstrained reachesthan in constrained reaches. Channel units in uncon-strained reaches had a greater percentage of area in lat-eral habitat subunits (P < 0.05) and had a more hetero-geneous distribution of subunit habitats. In September,when stream discharge was 18 cfs, habitat distributionwas expressed as the average percentage of channel unitarea within each reach:

Reach Type1Subunit Constrained Unconstrained

Hydraulic Jump 4.2

2.0Chute 12.4

9.9

Riffle 18.5 20.1Tranquil (Pool)

39.4

32.4Lateral Eddies and Backwaters2

9.6

6.2Exposed Channel

17.9

19.4

100.0

100.0

1 Constrained n=88, unconstrained n=78,2 P < 0.05, Kruskal Wallis test

Unconstrained reaches also had a greater diversity ofhabitats and increased availability of refuge habitat dur-ing floods. Equivalent measurements taken at higherflows (approximately 200 cfs), indicate that fast-waterhabitat subunits (hydraulic jumps and chutes) domi-nated the subunit distribution of channel units in con-strained reaches. A hydraulic jump is defined by thedominance of very turbulent, supercritical flow. Chutesubunits are areas of localized flow convergence, char-acterized by velocities at the threshold of supercriticalflow. Although the area of fast-water subunits also in-creased at high streamflow, in unconstrained reaches therelative distribution of subunits did not change and thearea of lateral habitat subunits increased nearly 60 per-cent.

In pool channel units, increased habitat complexityaltered the patterns of juvenile abundance when largerainbow trout were present. The structure of poolchannel units was variable both between and withinreach types. Habitat complexity in pools was generallygreater in the unconstrained reaches but complexity was

258 USDA Forest Service Gen. Tech. Rep. PSW-110. 1989.

strongly affected by the distribution and abundance ofboulders and large wood debris within the channel unit(fig. 4). In pools with low habitat complexity, juvenileabundance was lowest in the presence oflarge adult trout(p < 0.01, Kruskal Wallis test, fig. 5). In pools withhigh habitat complexity, juvenile abundance was similarregardless of the presence or absence of large adults.

10 meters

Discussion

Populations of juvenile cutthroat trout are apparentlyregulated by reach-level effects on stream habitat struc-ture and stream productivity. Habitat structure andproductivity, in turn, are both influenced by the geomor-phic structure and interactions between the stream, thevalley floor, and the adjacent hillslopes. The relation-ship between reach type and habitat structure (particu-larly lateral habitats) plays a key role in recruitment totrout populations. The strong correlation between lat-eral habitat area and number of age 0 cutthroat troutin a reach (r = 0.983; Moore and Gregory 1988b) un-derscores the importance of lateral habitats in the earlylife history of these fish. In the Cascade Mountains, cut-throat trout emerge during a period of declining stream-flow after the winter rainy season. however, velocity inmain channel habitats easily exceeds the swimming ca-pacity of 20-30 mm long cutthroat trout. If the marginsof the stream channel are abrupt and have either deepwater or fast current, juvenile cutthroat trout will bedisplaced downstream until they reach suitable habitats.In addition to the losses from a particular reach, down-stream movement also increases exposure to predatorsand potentially reduces the abundance of fry through-out the stream section.

Adult trout abundance was also related to reachtype, but was additionally influenced by the availabilityof channel units (pools) with complex structure. The

Figure 4 - Examples of pools with low (left) and i..:(right) levels of habitat complexity. Darker lines she....margins of the active channel, lighter lines show the ecl_of the wetted channel, boulders and wood. Both v.,:are located in unconstrained reaches of LookoutPools with lowest habitat complexity were similar to !:-

pool on the left but had fewer small boulders alongmargins.

40NMI FRY

0 = ADULTS

0E 30-0

0 20

0cc 10

0CASCADE RAPID RIFFLE POOL 2° CRAY,

CHANNEL UNIT

Figure 5 - Comparison of juvenile abundance in pool

with low and high levels of habitat complexity ai,

the presence or absence of large (length > 22 cn-.

rainbow trout. Habitat complexity was evaluated bycombination of subunit diversity, depth, and abundancof large boulders and wood. Error bars are +/- one SEN = 10.

25';?USDA Forest Service Gen. Tech. Rep. PSW-110. 1989.

maintenance of complex structure in stream channelsresults a the development of channel units with avariety of habitat subunits. The greater abundance ofcutthroat trout in the unconstrained reaches of LookoutCreek is apparently a function of the effects of riparianvegetation and geomorphic complexity on the structureof appropriate liabitats.

Because unconstrained reaches of Lookout Creek havea more open canopy, more light energy is available, andprimary production is greater than in the constrainedreaches (Gregory and others 1989). Greater light in-tensity and increased production in the unconstrainedreaches may, in addition to habitat effects, contribute tothe greater abundance of fish in these reaches. Previousresearch has demonstrated that cutthroat trout popula-tion density and biomass are greater in recently logged(open clearcut) stream sites than in streams with matureconiferous stands (Aho 1977; Murphy and Hall 1981;Murphy and others 1981). The abundance of macroin-vertebrates and the availability of drifting prey may begreater in reaches with open riparian canopies because ofthe effect of increased light on primary production, de-tritus quality, and prey capture efficiency (Gregory 1980;Wilzbach and others 1986). These studies have empha-sized the influence of riparian setting on the productionof food and related trout growth. Interpretation of theseresults, however, could be enhanced by an assessment ofhabitat structure and geomorphic setting.

Much of the organization of trout populations appearsto be derived from the effects of reach level geomor-phology on local levels of habitat. Intraspecific inter-actions play a smaller, but demonstrable role in regulat-ing the distribution of juveniles. Both of these processesare further modified by the effects of riparian vegeta-tion on habitat structure and food production. Thispattern is consistent with models of community regu-lation that incorporate variable levels of physical dis-turbance and abiotic control (i.e., Menge and Suther-land 1987). Stream systems provide gradients of phys-ical harshness and complexity of habitat structure thatare appropriate for analysis with the Menge- Sutherlandmodel (Schlosser 1982; Peckarsky 1983). In this study,gradients of physical harshness occur in at least twodimensions, longitudinal changes associated with reachtype and lateral differences in habitat structure. Uncon-strained reaches can be considered less harsh becausethe energy of flood events is dissipated across the broadvalley floor. The physical processes that regulate the ini-tial recruitment of trout result in greater abundance inthese areas. Following the establishment of an age class,however, habitat complexity, interactions with possiblepredators, and system productivity have subsequent in-fluence on the structure of populations.

The importance of valley floor habitats has been fre-quently demonstrated in large river systems with com-

plex fish communities. In large rivers, lateral habitats ofthe floodplain riparian zone include side channels, oxbowlakes, marshes, ponds, and tributary streams. Fish pro-duction and community richness are strongly correlatedwith the area of floodplain in a given reach (Welcomme1985). Channelization of these systems has resulted indecreased habitat heterogeneity, decreased production,and decreased trophic complexity as well as lower stand-ing stock and richness of fish communities (Horde andLake 1983). In Cascade Mountain streams, fish commu-nity structure is much less complex than in large, low-gradient, river systems. Floodplain geomorphology andlateral habitats, however, have an analogous function inthe structuring of fish populations. Community orga-nization in both large and small streams can be betterunderstood by considering the effects of the geomorphicstructure of channel units, the stream reach, and thevalley floor.

Acknowledgments

We thank Fred Swanson and Gordon Grant for theircontribution to the development of a geomorphic per-spective on stream structure. Joshua Lonquist and Ran-dall Wildman assisted in the fish census and habitatmapping. This research was supported by grant # BSR8508356 from the National Science Foundation.

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