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River, and, biotic factors such as competition are more directive in relatively benign and predictable systems (Poffand Ward 1990), maintaining natural variability is important in maintaining the native fish fauna in the Yampa River. Macroinvertebrate community structure is a product of the physical and biological influences in the environment. Gordon et al. (1992) described a number of elements that interact to give rise to the general character of a stream. These elements include elevation, stream order, length, discharge, slope, and substrate type. Macroinvertebrates have evolved anatomical, behavioral, and physiological characteristics to adapt to the conditions oftheir environment (Poff and Ward 1989; Poff and Ward 1990). Macroinvertebrate community structure is then limited by the physical and biological constraints of each species and the disturbances existing in their environment. The dominant force contributing to the structure of aquatic macroinvertebrate communities is dependent upon the time of year, adaptations ofthe given macro invertebrates, and magnitude of the disturbance (Poff and Ward 1989). The flow regime of a stream is usually considered to be one of the most important factors that influence aquatic communities (Poff et al. 1997). Flow is particularly important because it is often correlated with numerous other factors that influence the aquatic community (Poffand Ward 1990). These factors include: depth, velocity, thermal changes, renewal of resources, etc. Floods or high flows have been shown to be beneficial to disturbance dependent species, whereas other species thrive in a more stable environment (Poff and Ward 1989). Invertebrate communities in streams with a high degree of flow variation are typically dominated by species that can efficiently colonize new areas or can behaviorally avoid adverse conditions (e.g., invertebrates adapted for swimming). Streams with a low frequency of natural disturbance are generally inhabited by more diverse populations of invertebrates that often are larger, more specialized, and long- lived (Poff and Ward 1989). The macro invertebrate community composition of a given stream is adapted to, and dependent on, the flow regime (as well as other variables) existing in that stream. Changes in the natural flow regime of a system will result in changes to the aquatic communities (Poff et al. 1997). Some behavior of aquatic invertebrates has been shown to be dependent on flow. Manipulation of flow has been associated with an increase in drift of many species (White et al. 1981; Poff and Ward 1991). Poff and Ward (1991) indicated that the drift patterns of several species of macroinvertebrates in the upper Colorado River were influenced by increasing or decreasing stream flow. Several taxa responded positively (increased drift) to artificially lowering the flow. These taxa included Baetis sp., Triznaka 27