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Final Report 3.6.1 Channel Planforms 3-37 September 2000 As described in Section 3.3, the Green River downstream of Flaming Gorge Dam consists of a series of linked segments of three channel planform types without a systematic downstream change from one planform to the next. The channel planforms types are restricted meanders, fixed meanders, and canyons with abundant debris fans (Figure 3.15). These planforms are described below because the geomorphic processes and habitat conditions within each type can be quite different. 3.6.1.1 Restricted and Fixed Meanders Restricted meanders occur in broad alluvial terraces that are bounded by relatively more resistant geology (Figure 3.15). Valleys in which restricted meanders occur are relatively wide (greater than 1.5 km), and only the outside bends are in contact with bedrock. Restricted meanders occur in Reach 1 (Browns Park) and much of Reach 2. Fixed meanders are confined by resistant geology on both outside and inside bends and result from symmetrical incision associated with rapid down-cutting through the geologic formation (Figure 3.15). Labyrinth Canyon in Reach 3 is characterized by fixed meanders. Channel planform - The configuration of a stream as seen from above. Three channel planforms, defined below, are found along the Green River downstream of Flaming Gorge Dam and are shown in Figure 3.15. Restricted meander - Sinuous portion of river that flows through broad alluvial terraces bounded by relatively more resistant geology. Only the outside bends are in contact with bedrock. Fixed meander - Sinuous portion of river that is confined by resistant geology on both outside and inside bends. Typical elements of fixed and restricted meanders include the channel, vegetated islands, unvegetated bank-attached compound bars, unvegetated island-attached compound bars, and unvegetated mid-channel compound bars. Permanent islands are less common in fixed meanders than in restricted meanders. In-channel deposits are typically sand, although gravel bars sometimes occur. Typically, bank-attached compound bars occur on alternating sides of the river. Shoreward from these bars is the vegetated floodplain at the edge of the "bankfull" channel (Le., the channel that can accommodate stream flow without overtopping the banks), and streamward from the bars is the meandering thalweg. Island-attached compound bars are bounded by vegetated islands and the thalweg. Canyons with abundant debris fans - Relatively straight sections of the river confined on both sides by resistant geology with coarse sediment deposits (debris fans) at the mouths of tributaries. At low discharge, exposed compound bars have an irregular topography caused by chute channels that dissect the bar platform. Chute channels are oriented in a downstream direction, crossing from the streamward to shoreward side at the upstream end of the bar and from the