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Pools <br />As flows declined from 11,200 to 5,020 cfs, pool area decreased by half to a low of 5,300 m2. As <br />discharge continued to decline, pool area increased; at 1,630 cfs, total pool area was three times <br />what it was at 5,020 cfs. However, at still lower flows, pools again decreased in area and were <br />back to about 5,900 m2 at 810 cfs. At very low flows (557 cfs), velocities are significantly reduced <br />and many portions of slow runs were transformed into pools; by definition, pools are any within- <br />channel habitats with a mean-column velocity < 0.35 ft/sec. As a result, pool area dramatically <br />increased at very low discharge. <br />Slow Runs <br />Slow runs were the dominant habitat type at discharges of 4,400 cfs or less. As flows continue to <br />decline, slow run area gradually increases to a high at 1,240 cfs. At still lower flows, some slow <br />runs are transformed into rifliles or pools and total slow run area decreases. However, at even the <br />lowest flows, slow runs make up over 50% of the total water area. <br />Fast Runs <br />With rising flows, fast runs increase in area in a consistent manner. The higher the water, the <br />greater the area of fast runs. Above 4,400 cfs, fast runs become the dominant habitat type (46-60% <br />of TWA). At flow levels of 1,630 cfs or less, fast runs make up 5-20% of the TWA. <br />Rimes <br />Change in the quantity of riffle habitat occurs in just the opposite sequence as that of backwaters <br />and eddies. Riffle area was lowest at 7,620 cfs and then almost tripled as flows declined to 4,426 <br />cfs. As flows further declined by 87%, from 4,426 to 557 cfs, total riffle area never decreased by <br />more than 21 %. As a percent of total habitat, riffles consistently comprised 20-27% of the total <br />water area at flows of 4,426 cfs or less. Thus, riffles remain relatively constant and fairly abundant <br />at all moderate to low flows. <br />Rapids <br />Rapids occurred at all flow levels in Site No. 1 only. In the other three sites, rapids occurred only <br />when discharge exceeded 4,426 or 5,020 cfs. Total rapid area peaked at 7,900 m2 when discharge <br />was 4,426 cfs, and decreased at higher flows presumably as they were transformed by deeper water <br />into fast runs. At the lowest flows, rapids disappeared almost entirely. <br />Change in Habitat Composition <br />During the higher discharge levels (5,020-11,200 cfs), fast runs were the dominant habitat type, <br />comprising 46-60% of the total water surface area (Fig. 12). At flows less than 5,020 cfs, fast run <br />area tapered off and slow runs (40-6000) became the dominant type. At all discharge levels less <br />than 2,870 cfs, riffle habitat was the next most abundant type (21-27%) after slow runs. The other <br />five habitat types that we mapped were much less abundant and made up a relatively small part of <br />26