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10 <br />and the alluvial valley is wider than 1.5 km (Fig. 3a). Fixed meanders occur elsewhere, <br />except in canyons with abundant debris fans. <br />The Distribution of Types of Low-Velocity. Habitat in Each Channel <br />Planform Type <br />Restricted and fixed meanders: Restricted (Fig. 6) and fixed meandering <br />(Fig. 7) reaches are composed of the channel, permanent vegetated islands, unvegetated <br />bank-attached compound bars, unvegetated island-attached compound bars, and <br />unvegetated mid-channel compound bars; permanent islands are rare in fixed <br />meandering reaches. Thalweg and bar substrate is typically sand, although gravel bars <br />occur in some segments. Typically, bank-attached compound bars occur on alternating <br />sides of the river, shoreward from these bars is the vegetated floodplain at the edge of <br />the bankfull channel, and streamward from the bars is the meandering thalweg. Island- <br />attached compound bars are bounded by vegetated islands and the thalweg. <br />At low discharge, exposed compound bars have an irregular topography caused by <br />chute channels that dissect the bar platform (Fig. 8). Chute channels are oriented in a <br />downstream direction, crossing from streamward to shoreward side at the upstream end <br />of the bar, chute channels cross from shoreward to streamward side at the downstream <br />end of the bars. Topography is more complex where there are more chute channels. At <br />some sites and in some years, secondary bars become attached to the shoreward margins <br />of these compound bars (Fig. 7). At the downstream end of most compound bars, chute <br />channels may converge into one persistent and deep secondary channel that separates the <br />downstream end of the compound bar from the floodplain (Fig. 7 and Fig. 8). This <br />secondary channel is typically so deep that it constitutes a backwater habitat even at <br />very low discharges. The remainder of the bars are composed of broad, level platforms <br />and linear ridges that may be partly vegetated.