Stream: South Fork Slater Creek (Lower Segement) Executive Summary

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instream flow recommendation from the SF Slater Creek upstream if the confluence with West Prong South Fork Slater Creek (4.1 cfs) to recommend a 9.0 cfs instream flow during the summer months, The winter flow recommendation, which meets 2 or 3 criteria and is within the accuracy range of the R2Cross model is 5.25 cfs. These recommendations were derived by averaging the results of the two data sets. Hydrologic Data and Analysis After receiving the cooperating agency's biologic recommendation, the CWCB staff conducted an evaluation of the stream hydrology to determine if water was physically available for an instream flow appropriation. This evaluation was done through a computation that is, in essence, a "water balance ". In concept a "water balance" computation can be viewed as an accounting exercise. When done in its most rigorous form, the water balance parses precipitation into all the avenues water pursues after it is deposited as rain, snow, or ice. In other words, given a specified amount of water deposition (input), the balance tries to account for all water depletions (losses) until a selected end point is reached. Water losses include depletions due to evaporation and transpiration, deliveries into ground water storage, temporary surface storage, incorporations into plant and animal tissue and so forth. These losses are individually or collectively subtracted from the input to reveal the net amount of stream runoff as represented by the discharge measured by stream gages. Of course, the measured stream flow need not be the end point of interest; indeed, when looking at issues of water use to extinction stream flow measurements may only describe intermediate steps in the complex accounting process that is a water balance carried out to a net value of zero. In its analysis, CWCB staff has attempted to use this idea of balancing inputs and losses to determine if water is available for the recommended Instream Flow Appropriation. Of course, this analysis must be a practical exercise rather than a lengthy, and costly, scientific investigation. As a result, staff has simplified the process by lumping together some variables and employing certain rational and scientifically supportable assumptions. The process may be described through the following description of the steps used to complete the evaluation for this particular stream. The first step required in determining water availability is a determination of the hydrologic regime at the Lower Terminus (LT) of the recommended ISF reach. In the best case this means looking at the data from a gage at the LT. Further, this data, in the best case, has been collected for a long period of time (the longer the better) including wet and dry periods. In the case of South Fork Slater Cr - Lower no such gage is available at the LT. In fact, there is no gage on South Fork Slater Cr - Lower. It is thus necessary to describe the normal flow regime at Chaparral Creek above the LT through a "representative" gage station. The gage station selected for this purpose was NORTH FORK ELKHEAD CREEK NEAR ELKHEAD, CO (USGS 09245500); it has a period of record (POR) of 15 years collected between 1958 and 1973. The gage is at an elevation of 7,005.00 ft above mean sea level (amsl) and has a drainage area of 21.0 mil. The hydrograph (plot of discharge over time) produced from this gage includes the consumptive uses of numerous diversions. However, the existence of these diversions does not preclude use of the data from the gage. To make the measured data transferable to South Fork Slater Cr - Lower above the LT, the consumptive portions of these diversions were added back to the measured hydrograph. The resulting "adjusted" hydrograph could then be used on South -4-