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CHAPTER VI INTRODUC1I10N TO STREAM CLASSlFICATION WHY CLASSIFY COLORADO STREAMS There are three purposes in classifying stream channds within th e State of Colorado as part of the development and funding of rehabiliultion projects, First, the stream classification methodology should serve as an indicator of the dominant physical processes present in the river system of interest. Second, the classification methodology should aid in the judging of competing restoration projects, Third, the classification sbould allow for clear communication betwel:n all parties involved in restoration projects for design information and cost, UNDERSTANDING THE RIVER ENVIRONMENT Rivers are dynamic features of the ealth's surface that change over time, A basic understanding of the physical processes that are responsible for river form is needed in order to work within the river environment. Unlike static features on the earth's surface, such as a road, there is no template for the construction of a river. The geometry of a river is self adjusting in response to the water and sediment that is supplied to them. Over time, changl$ in water and sediment supply result in channel adjustment. Rivers are never in equilibrium or rigid, Consistency of form best describes a "stable" river pattern, although river form often changes due to various natural constraints found in the physical and ecological setting of a river system, The essential governing variables of the riv(:r pattern are the water discharge and sediment load with stream gradient and bed-material composition further characterizing the power and mobility of the stream channel. The relationship of these governing variable to one another was summarized by Emery Lane in 1957 in the following relationship: GbDb - Q 5 EQ,1O The product of discharge, Q, and stream gradient, 5, is the power available within the stream to transport the supply of water and sediment within the stream channd. The mobility of the channel bed depends on the supply of sediment, particularly the sediment that is in motion from the channel bed, Gb, and the characteristics size of the sediment particles the compose l:he channel bed, Db' As a river increases in power and mobility it becomes le:;s predictable. Figure 1 illustrates the increasing uncertainty and risk that occurs with increasing power and mobility, In is important in the rehabilitation of stream channels that the physical dif11culty and uncertainty associated with the work be understood and Vl-l 97-Q60,OOI