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The process of gamete development, migration, staging, egg ripening and spawning <br />occurs over an extended period and each phase is timed to roughly coincide with <br />predictable phases in the runoff cycle. The duration of these runoff phases may therefore be <br />as important as the magnitude of runoff. Maintaining a semblance of the historical duration <br />of the runoff period may therefore be necessary for this species to successfully complete its <br />annual reproductive cycle. <br />HYDROLOGY <br />The flow regime of the Colorado River has been significantly altered as a result of <br />water development. Understanding the effects of water development on endangered fish <br />and their habitats requires an evaluation of the manner in which this alteration has occurred. <br />Previous attempts to do this have relied on comparisons between mean monthly flows (and <br />mean annual peak flows) of a `pre-development' block of years with those of a `post- <br />development' block of years (Osmundson and Kaeding 1991, Pitlick et al. 1999). However, <br />accurate quantification of effects from regulation on discharge is difficult because of (1) <br />changes in climate between periods and (2) inadequate samples (years) that truly represent <br />pre- or post-development periods. Consequently, results can vary greatly depending on the <br />years selected for comparison. Fortunately, for the Palisade-to-Rifle reach, these potential <br />sources of bias can be avoided by using a data set developed by the Natural Resources <br />Conservation Service (MRCS). The NRCS investigators estimated, to the extent possible, <br />what the monthly inflow at the USGS gauge near Cameo would have been in the absence of <br />regulation. They did this by examining monthly storage and diversion records for each year <br />during 1958-1997 (ungauged diversions could not be accounted for). For this report, the <br />unregulated monthly yield values (acre feet) provided by NRCS were converted to mean <br />monthly discharges (cfs); these were then compared with actual discharges recorded at the <br />USGS gauge. Hence, the same block of years was used in comparing regulated with <br />unregulated flows. <br />31