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of the water supply. The mean annual flow might indicate a plentiful water supply but, due to the "feast and famine" delivery pattern, the aquatic community cannot take full advantage of the available supply. The second problem is that increased peak flows tend to enlarge the channel, magnifying the problem of diminished base flows. b. Sediment yield. Although the flow regime essentially determines the size of a channel, the amount of sediment reaching the stream and the ability of the stream to transport sediment jointly determine its shape. If the sediment load supplied to the stream is balanced by the ability of the stream to transport that load, no net change in channel structure occurs over time, and the stream and watershed are said to be in equilibrium. However, if the watershed contributes more sediment than can be transported by the stream, the channel structure changes to a more efficient shape for transporting sediment. This shape is usually wider and shallower than the original channel and, in extreme cases, the stream reverts from a meandering or riffle-pool sequence to a braided channel. This process is called channel aggradation. The channel response is just the opposite if the watershed contributes less sediment than can be transported or if the sediment is intercepted (as in a reservoir). This is termed channel degradation. These changes affect the microhabitats of fish, invertebrates, and even aquatic vegetation. The most obvious effect in many streams is a change in the particle size of the bed material. The bed material frequently becomes smaller in aggrading channels. In degrading channels, the bed material gets larger because the finer material is swept away. Redistribution of hydraulic characteristics of the discharge (depth and velocity) usually accompanies a change in channel shape and particle size. c. Chemical yield. Water chemists classify the sources of chemicals entering the waterway as point loads and nonpoint loads. The watershed and the associated land use determine the nonpoint sources. Unlike sediment, chemicals may enter an open channel either through surface runoff or ground water inflow, with certain chemicals often predominantly associated with one mode of entry or the other. The distinction between point and nonpoint sources, and surface or ground water entry, may be very important to an instream flow study because the concentrations of some water quality constit- uents may be established before the water reaches the stream. Changing the concentrations of these constituents may not be possible by manipulating the streamflow, particularly if the entire water supply for the stream originates within one homogeneous watershed. Such water chemistry problems are related, but independent of streamflow, and cannot be resolved solely by an instream flow study. The water chemi-stry of a stream in an undisturbed watershed generally reflects a homeostasis between streamflow and nonpoint chemical yield. The primary water quality concerns will probably be with point sources, many of which can be at least partially mitigated by water supply manipulations. However, changes in land use on the watershed can result in accelerated chemical loadings, only some of which can be resolved by increasing the stream- flow. Runoff from feedlots or agricultural lands can increase the loading rates of nutrients and oxygen consuming compounds. Although these are nonpoint 8