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I I I I I I I I I I I I I I I I I I I 4.2 4.3 Bank erosion impacts sediment continuity response by contributing large volumes of sediment in short periods of time in localized areas. The sediment analysis is further complicated by the fact that bank erosion and lateral migration are not continuous throughout the study reach. The processes are limited in several locations by vegetative cover, or the lack of sufficient shear stress to remove intact bank material. Recognizing the complexity and diversity of the lower Elk River system, a limited investigation of channel stability was accomplished by the following process: 1. Qualitative geomorphic analysis. This included a geomorphic interpretation of both field reconnaissance information along the study reach and of historic information regarding channel behavior. Results from this analysis provided a foundation for the alternative analysis. 2. Bank erosion and lateral migration analysis. This included evaluation of both qualitative and quantitative elements of historical and potential bank erosion which were then considered in the alternative analysis. BED AND BANK MATERIAL COMPOSITION No sediment sampling was performed as part of this study. Field observation along the lower reach of the Elk River indicated that bed material was primarily cobbles mixed with sandy gravel. Bank material appeared to vary, but seemed to be predominantly a mixture of loam, sand, and gravel with some cobble present. GEOMORPHIC ANALYSIS 4.3.1 Purpose and Data The geomorphic evaluation defines the characteristics of the lower Elk River based on field investigation, available data, and mapping. The qualitative analysis involved collecting reconnaissance level field data for channel bed and banks, evaluating general historic changes in planform, sediment production zones, local geology, and geomorphic trends. The analysis included evaluation of aerial photographs and historic land use patterns, and application of relationships for basic geomorphic concepts. Results provided a geomorphic basis for use in evaluating stabilization alternatives in the study reach. 4.3.2 Geologic Setting Channel bank material reflects the underlying geology for the study reach. Geologic mapping for the area (USGS) indicates alluvial deposits of sand, silts, and gravels along the lower Elk River corridor. The initial geomorphic reconnaissance supports that the material is consistent with this description. Evidence of the Mancos Shale formation along the eastern channel bank that appeared to be acting as a geologic control was noted during the geomorphic reconnaissance. Field observations suggest there is some seepage from the banks into the channel, and that a relatively high groundwater table exists along much of the study reach. Field observations appeared to correlate with the soil survey information for Routt County pertinent to this area that was provided by the NRCS. 24