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patterns. The relatively unregulated Yampa River exhibits great seasonal flow variability, with a <br />20-yr mean flow peak (1979-1999) of about 223 m'/sec, and an associated base flow (August- <br />February) of about 10 m'/sec (Fig. l ). Mean daily flows during the summer (15 June to 15 <br />September) were 43.7 m'/sec from 1979-1999. During the recent drought period of 2000-2006, <br />annual peak flow exceeded the 20-year average peak in all years except 2002 and 2004, but base <br />flows were much lower and began earlier (Figs 1-3). Flows during summer (15 June to 15 <br />September) averaged only 17.5 m3lsec from 2000-2006 and generally reached seasonal lows by <br />around the first of July rather than the first of August. An exception to this was 2005, when the <br />onset and level of base flows closely mimicked the pre-drought 1979-1999 average. <br />Lower flows, and likely warmer air temperatures, in the 2000-2006 drought period, <br />resulted in higher water temperatures in the Yampa River study area. For the period 1991-1999 <br />(no data for 1996 were available), water temperatures warmed earlier (mid May, Fig. 4) than in <br />the drought period 2000-2005 (a full summer of 2006 water temperatures was not yet available), <br />and were substantially higher in summer, particularly June and July. For example, mean daily <br />water temperature from 15 June to 15 September (a main period for fish growth) from 1979- <br />1999 was 19.0°C compared to 20.5°C in 2000-2005. In every year in the 2000-2005 period, <br />mid-June to late July water temperatures were substantially warmer and warmed earlier than in <br />the 20-year 1979-1999 pre-drought period (Figs. 4-6). For example, average water temperature <br />of the Yampa River was sustained at 16°C in the 1979-1999 by 25 June, whereas in the 2000- <br />2005 drought period, water temperature was sustained or above 16°C by 14 June, 8 June, 29 <br />May, 26 June, 14 June, and 22 June, in the years 2000 to 2005, respectively. Lower <br />temperatures later in the year in 2003 and 2005 were associated with relatively high flows later <br />in the year. <br />Yampa River habitat sampled.-A total of 257 individual habitat units was sampled <br />during this study, 249 of which supported fish (Table 2). Number of habitat areas sampled was <br />relatively low in 2004 because of effort expended in gear sampling efficiency evaluations and <br />because of high flows through most of October. A few additional samples were collected with <br />8 <br />