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• been channelized and straightened. This was probably initially done as part of tht: construction of the agricultwal irrigation ditch infrastructure, and later influenced by the construction of the railroad and highway to the north of the channel. Several low-profile lineaz berms along the south bank of the main channel, and the non-sinuous channel alignment itself aze evidence of the channelization. The straightening of the channel has increased the channel's grade and therefore the velocity of flows. This has created what is considered to bean "unstable" alignment, with the river exhibiting a tendency to lower its thalweg through degradation. This is true along much of the St. Vrain in Boulder County, and is evidenced by moderate to severe bank erosion found throughout the river reach from Lyons to Longmont. It should be noted that this is a long-term instability that only becomes evident after many yeazs, but will continue to occur as the. river seeks to reach a state of equilibrium. As discussed later in this report, it is important to take into account the potential instability of the river in the final design of the phased reclamation ~~lans for the proposed mining operation. The banks of the Si. Vrain adjacent to the proposed and existing mining operation aze generally well vegetated, and lined with brush and trees. The channel is irregular in cross section, with the left {north) streambank being considerably higher and steeper than the right (south) streambank. Some scow and bank failwes are evident on the steeper north bank, however the south bank is well vegetated and stable. • Current floodplain maP4ing The current regulatory floodplain model for the St. Vrain Creek is the Colorado Water Conservation Boazd, North St. Vrain Creek Unstream of Lyons ~& St. Vrain Creek Downstream of Lyons, West of Lanemont. Boulder County, Colorado, Flooded ,areas prepazed by Engineering Professionals, Inc. with floodplain re-analysis and floaiway delineation by Love and Associates, Inc, October 1992. The model was adopted by resolution in 1992 by both the Boulder County Board of County Commissioners and the State of Colorado Water Conservation Board. The model has not yet however, been adopted by the Federal Emergency Management Agency and therefore the floodplain limits shown on National Flood Insurance Program Flood Inswance Rate Map Panel 255 of 595, Map number 0801300255 are based on the older U.S. Department of Army, Corps of Engineers, Omaha District, Flood Plain Information, Lower St. Vrain, Volume III, Boulder County. Colorado, June 1972. On St. Vrain Creek between U.S. Highway 36 and North 75m Street, the hydraulic analysis is divided into two floodplains: main channel and south floodplain. Between U.S. 36 and North 51°` Street (Southdown entrance), flow spills from the main channel to the south because the conveyance capacity of the main channel is inadequate to convey the full 100-yeaz flow. A 100-year flow split of approximately 4,000 cfs is estimated to be conveyed in the south floodplain. The 100-year flow conveyed by the main channel • downstream (east} of North 51" Street is approximately b,000 cfs. At several laations