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HABITAT AREA AS A FUNCTION OF DISCHARGE (Objective No. I) Change in Area of Mesohabitats General We mapped habitats in the study sites at 11 different discharge levels: five during 1990; six during 1991 (Figs. 8 and 9). Discharge ranged from a low of 557 cfs to a high of 11,200 cfs, as measured at the top of the 15-mile reach. With some minor exceptions, increases or decreases in the area of a particular habitat type from one flow level to the next occurred in a reasonably predictable manner. Examples of maps of one study site at two different flows is provided (Fig. 10). Flooded Gravel Pits The one flooded pond made up a total of 15% of Site No. 3 and 2.5% of the four sites combined during discharge levels greater than 9,000 cfs. At lower flows, its percent contribution was less. At flows of 2,870 cfs or less, the pond was isolated from the river (Fig. 11). Unlike backwaters, gravel pits continue to enlarge with increased flow. At very high flows, over-bank flooding would create additional low-velocity, inundated, floodplain habitat types. Such high flows did not occur during the mapping years of 1990-1991. A few gravel pits, such as the one in Site No. 3, communicate with the river at relatively low spring flow levels. Most along the river are separated by high dikes. Because over-bank flooding occurs much more infrequently than was the case during historic flow conditions, these few connected gravel pits may function as an ecological surrogate for once common inundated, floodplain habitats. Backwaters Total area of backwater habitat peaked when discharge was 7,620 cfs and decreased at higher discharge levels (Fig. 11). Most backwaters in the 15-mile reach are created in side channels that cease to flow during low water. The upper end goes dry and is cut off from the main channel; at the lower end, water from the main channel backs up into the mouth forming a zero-velocity habitat. These backwaters continue to increase in size with flow until the river tops over the bar at the upper end of the channel and the side channel begins to flow again. As flows increase, some backwaters are lost to this process while others are still becoming larger. At the same time, still others may be just beginning to form as less-frequently, flooded channels or other low-lying features become inundated. At the eleven flow levels we studied, more backwater area was lost than created at flows greater than 7,620 cfs. Backwater area also decreased as flows were reduced below 7,620 cfs. As flows dropped below 1,530 cfs, additional side channels dried up at the upper end forming backwaters at the lower end. This created a spike of increased backwater area at 1,240 cfs. At flows less than this, backwater area again decreased as the main channel lost its ability to keep the mouths of chute channels inundated. 22