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colonization was highest on substrates with particle sizes of 25-35 mm and lower on smaller sizes and considerably lower on larger sizes. However, they found that insects may colonize small substrata (10-35 mm in diameter) because these serve as a better food collecting device than do larger or smaller substrata and that manipulation of the substratum may alter stream productivity through an influence on its detritus storage capability. They also reported that the substratum-detritus interaction was the overriding influence on insect microdistribution and that current velocity and a light deposition of silt play only secondary roles. They concluded that detritus was of primary importance but that it is the physical factor of substrate particle size which determines the distribution of the detritus. They state: "Current velocity interacts with all other distributional factors. It sorts and separates the substratum particles, deposits and carries away silt, transports and shreds leaf detritus, and is necessary for organism respiration. The substratum particles act as detritus collectors. Small particle sizes tend to accumulate large amounts of the small detrital particles, which are primarily leaf fractions. This type of detritus does not accumulate in the larger-size substrata, where the interstices are larger and current velocity is higher." In the Colorado River, organic input may be seasonally important when high runoff flows entrain shoreline debris into the channel where it can be processed by invertebrates. Later in the season, allochthonous input is reduced and may be of lesser importance as a food web energy source. Primary productivity via the production of periphyton on rock surfaces may then assume greater importance. Rock size might be of less importance at such times since it would be the surface area exposed to light rather than the surface area of the interstitial voids that would place a limit on the availability of food for invertebrates. If so, the former situation (allochthonous input) would probably be the more sensitive to a reduction in depth- to-embeddedness from the deposition of fines. Surface area for algal attachment on rock surfaces exposed to light will be little affected unless embeddedness is so severe that the tops of rocks are covered. However, high embeddedness without the loss of the top surfaces could still result in the loss of invertebrates through the loss of their protected living spaces. Clearly, more needs to be learned regarding the functional groups of aquatic invertebrates inhabiting this part of the Colorado River and the relative importance of autochthonous and allochthonous energy inputs before the biological effects of changes in depth-to- embeddedness can be adequately interpreted. Streambed Disturbance Effects The life history attributes of invertebrates in river reaches characterized by scouring of the bed from runoff and spates is likely quite different than in lotic environments with a more stable and benign flow regime. Ward et al. (1986) believed that abiotic rather than biotic factors have had the greatest effect in shaping the zoobenthic communities of middle and high elevation Colorado River basin reaches. The frequency of bedload transport is probably of critical importance for organisms dependent on the benthos for cover, foraging, or attachment sites. Favored species will be those adapted to these predictable events. Rapid development and lack of dormant stages are adaptations that would allow successful completion of the life cycle before the next disturbance event. 27