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
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