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3 <br />A flood on June 18, 1965 was caused by a 46,600 cfs discharge from the Bijou Creek watershed. <br />The 1965 flood waters were described as a mile wide in the project area and so deep that only the <br />top few feet of each of “rainbow arches” of the Colorado Highway 52 bridge were visible. At that <br />flood stage, the water would have been about 20 feet deep. The owner of the property said the <br />1965 flood inundated the floodplain where the buildings and office are today with eight to ten <br />feet of water. <br /> <br />2.4.3 River Channel Function and Processes <br />The River is a substantially braided, low gradient system with high bedload. Many of the River <br />banks are eroding and sand-bed tributaries no doubt contribute large quantities of sediment to the <br />system during flood events. Upstream diversions that irrigate tens of thousands of acres of the <br />adjacent semi-arid landscape deplete the system’s normal flow regime, which shaped the River <br />over thousands of years. Today’s depleted flow regime contains far less, and likely inadequate, <br />frequent flow energy to transport sediment loads produced by occasional large magnitude events <br />from the system’s tributaries. The depleted flow regime has likely shifted the sediment/energy <br />balance to an excess sediment condition. <br /> <br />Aerial photographs of the project reach covering 1963 to 2006 (Figures 11-18) demonstrate the <br />rapid channel adjustments that occur in the River system. In 1963, the River exhibited typical <br />conditions including in-channel sandbar features and a vegetated riparian corridor. The 1965 <br />flood created vast, barren sandbars where none existed in 1963 prior to the flood, and <br />substantially widened the active channel by eroding the formerly vegetated north floodplain. Four <br />years later, in 1969, these scars were still clearly visible and still unvegetated. Since then, the <br />River has worked and reworked the sand and gravel deposits to create and again consume gravel <br />bars, side channels and entire islands. There are presently extensive stretches of eroding bank that <br />are active components of the on-going channel adjustment process. A comparison of the 1963 <br />and 2006 photos (Figures 11 and 18) reveals that the River is still in a very dynamic adjustment <br />condition and has a long way to go before it returns to the 1963 conditions. Large in-channel <br />sediment features indicated by the braided channel upstream of the project reach in the 2006 <br />photo will likely move into and through the project reach in future years. This 'slug' of material <br />may have adverse affects on the project reach as it tries to pass through the reach and through the <br />Highway 52 crossing. <br /> <br />Today’s left bank impact area is a large sand and gravel bar that exists slightly above bankfull <br />elevation. This area is part of, and just downstream of, an outside meander bend. As a result of <br />it’s location on the meander bend, the high flows in 2009 transported and deposited a significant <br />amount of sand and gravel material onto this impact area (Photo 11). Topographic survey of the <br />site and comparison photos taken before and after the 2009 runoff indicate that this year’s sand <br />and gravel deposits are one to two feet deep. <br /> <br />Given the line of attack of the flow around the meander bend, the magnitude and duration of flow <br />in 2009, and the spread of sediment-laden overbank flow, the deposition on the left bank impact <br />area is no surprise and part of normal channel processes in that reach. Old aerial photos show <br />this deposition area to be part of a remnant overflow side channel that formed between 1979 and <br />1989 and has been in the process of becoming abandoned as it fills with sand during over-bank <br />flow events. Abandonment of the side channel is occurring as the River creates a single thread <br />channel to the south with a bankfull width of about 142 feet. Examination of the 45-year aerial <br />photo record indicates that the processes observed on this left bank area appear to be part of the <br />adjustment process the River is making in response to the 1965 flood event. <br /> <br />The 2009 spring runoff event was of sufficient magnitude to transport and deposit sand and small <br />gravel above the bankfull elevation on the left bank impact area. This deposited bar material may <br />become colonized with upland riparian species and continue to form a new floodplain. The 2006 <br />photo (Figure 18) shows that some colonization had already occurred near the OHWL prior to this <br />year’s deposition.