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December 2002 FLOW-SEDIMENT EFFECTS ON RIVERINE FISH affected by sedimentation were those most specialized to feed from the substratum; in their study, both benthic insectivores and herbivores decreased in abundance as the percent of fine substrate increased. Our results sup- port this last observation: native suckers were generally more abundant in riffles (the more sediment-free hab- itat) than in runs. Nearly equal usage of the two habitat types by suckers in stratum 10 and 11 suggest they are not tied to riffles by some physical habitat preference. Riffles were likely more attractive than runs in most strata because of higher periphyton biomass (down- stream of stratum 9), invertebrate biomass, and detritus biomass-variables associated with cleanness of the bed. Flood-flow aspects Considerable research has focused on the role of physical disturbance from floods in structuring stream benthic communities (Cobb et al. 1992, Scarsbrook and Townsend 1993, Death and Winterbourn 1995, Poff and Allen 1995, Clausen and Biggs 1997, Blinn et al. 1999). Invertebrate density is reduced by flood disturbances (from shear stress removal and abrasion from moving particles), but these reductions are short-lived with numbers recovering in a few to several tens of days (Scrimgeour et al. 1988). Communities controlled by stochastic factors such as floods are characterized by high species turnover and rapid recolonization (Lake and Doeg 1985). However, rapid restoration after floods may be slowed in highly modified systems where alien assemblages adapted for more stable flow regimes have replaced native, fauna, such as in the Grand Canyon downstream of Glen Canyon Dam (Shannon et al. 2001). Clausen and Biggs (1997) reported increases in invertebrate density with low to moderate flood fre- quencies (several events per year). Our study suggests a mechanism by which low flood frequencies (<1 flood/yr) in a partially modified system might depress invertebrate production; i.e., the accumulation of fine sediment in upper layers of the bed. This effect, in mid reaches of the upper Colorado River, might be expected in other systems where impoundments are located up- stream of sources of fine sediment, resulting in a flow volume reduction without a corresponding reduction in fine sediment input. Constraints of the physical environment Standing crops of biota in the upper Colorado River are limited by characteristics of the physical environ- ment. Some constraints are relatively permanent, while others are more subject to short-term external change. Influences of physical attributes of the river can be viewed as hierarchically organized within a spatiotem- poral framework (Frissell et al. 1986, Scarsbrook and Townsend 1993). Within our study area, setting and morphology of the channel, dictated by geology and landform type (directional orientation, canyon vs. al- luvial valley, etc.) affect depth and shading and con- 1735 strain primary producers and consumers in each stra- tum to varying degrees. Overlain on this templet are the physical attributes of the riverbed. These attributes, dictated by sediment input and flow regime, also affect algal and macroinvertebrate production. Finally, water quality or clarity add yet another layer of potential constraint. Turbidity in the upper Colorado River varies seasonally (spring runoff), daily (summer thunder- storms), and longitudinally (irrigation and tributary in- puts). Dominant channel features set by landform and ge- ology, such as the deep water and steep banks of con- fined canyon stretches (strata 2, 3, and 5), are long- term attributes with little potential for change. Coarse sediment inputs from tributaries and channel margins are also relatively long-term constants. Inputs of fine sediment, on the other hand, may have increased in the Colorado Plateau watershed during historic and recent times as a result of land-use practices (e.g., grazing, irrigation, road building, off-road vehicle use); such potential changes are difficult to quantify, and are in need of further study. The transport and distribution of sediment once it has entered the river is determined by the annual flow regime, and bed conditions are there- fore subject to the short-term vagaries of both climate and human activities. From an applied perspective, the greatest opportunities for enhancing within-channel productivity appear to be through this flow-sediment link. To what degree fine sediment may influence standing crops of benthic biota in strata 10 and 11 is difficult to discern because of the relatively clean conditions we observed there during our period of study. In these two strata large loads of fine sediment begin entering the river via small tributaries that drain erodible low- elevation watersheds, increasing suspended sediment loads of strata downstream. Thus, strata 1-9 are prob- ably the most susceptible to problems associated with fine sediment accumulation. However, the bed of low- gradient stratum 1 has probably always consisted of sand and silt, and the productivity of strata 2-5 has likely always been constrained by the overriding lim- itations of channel morphology that limit autotrophy. The best opportunity for increasing main-channel food production appears to be in strata 6-9 and perhaps 10 and 11 as well. Flow alteration effects Periodic movement of surficial framework particles is required to remove fines below the surface layer (Millions 1973, O'Brien 1987, Kondolf and Wilcock 1996, Wilcock et al. 1996). Suspended sediment makes up more than 98% of the total sediment load in our study area (Butler 1986, Van Steeter and Pitlick 1998). At low flows, the immobile surface layer acts as a sink for suspended sediment, which deposits in the inter- stices. The bed thus acts as a silt and sand reservoir. At high discharge, when the surficial gravel-cobble