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Last modified
7/14/2009 5:02:33 PM
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UCREFRP
UCREFRP Catalog Number
8208
Author
Osmundson, D. B., P. Nelson, K. Fenton and D. W. Ryden.
Title
Relationships Between Flow and Rare fish Habitat in the '15-Mile Reach' of the Upper Colorado River.
USFW Year
1995.
USFW - Doc Type
\
Copyright Material
NO
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estimates of stream flow were calculated based on a formula that combines the readings of the <br />upstream Cameo and Plateau Creek gages and then subtracts stream flow diverted between those <br />gages and the top of the reach (see Osmundson and Kaeding 1991). Together with personnel from <br />USGS (Dan Collins, Sub-District Chief, Western Slope Sub-District) we plotted stream flow based <br />on the gage readings against stage elevation at three main channel (single channel) transects in the <br />15-mile reach. This was done on logarithmic graph paper. A french curve was then used to <br />correct inconsistencies in gage readings or gage and diversion-based, calculated stream flows. This <br />also helped smooth out any differences between reported mean daily flows and the flows during <br />that part of the day specific to when the mapping was being done. <br />Discharge varies among sites within the reach. Due to groundwater seepage and numerous <br />irrigation return canals, flow increases progressively downstream. During the imgation season, <br />flow at the bottom of the reach can be 150-400 cfs (averaging 200 cfs) higher than at the top <br />(Roush 1994). Due to time constraints while mapping, actual flow at the study sites could not be <br />measured. We used the flow at the top of the reach, either measured or calculated, as a consistent <br />surrogate for flow at the sites. Thus, to produce reported conditions at the sites or to recommend <br />flows for the reach it is assumed that discharge at the top of the reach is supplemented with the <br />current level of inflows unaccounted for here. <br />Analysis <br />Habitat Mapping Corrections <br />Inconsistent scale of the aerial video prints resulted in erroneous habitat-area calculations during <br />the original MIPS analysis. Problems were detected when it was noted that total water area (TWA) <br />did not always decrease with declining flows; in many instances TWA increased when it should <br />have decreased. Accurate scale is critical for this type of study because small differences in scale <br />will necessarily result in measured differences in habitat area among dates that will erroneously be <br />attributed to changes in flow. When the 1990 and 1991 data were combined and arrayed by dis- <br />charge level, from high to low, TWA would increase at many 1990 data points even though the <br />flow level was lower than for the preceding 1991 data point. As it turned out, there were several <br />differences in how BR had both acquired and analyzed the data between years. During the 1990 <br />videotaping, the helicopter was flown at a constant elevation as measured by a barometric altimeter <br />and the scale was calibrated to the lengths of nearby bridges. In 1991, the helicopter was equipped <br />with a radar altimeter and the scale was calibrated to flying height. Correction factors were calcu- <br />lated for the 1990 data to bring it more into line with the technique used in 1991. Although this <br />helped to some extent, problems still persisted. To reduce scale problems further we developed <br />habitat area correction factors for each flight date at each study site. We did this by plotting TWA <br />versus discharge and fit a regression line to that relationship. The predicted TWA for a given dis- <br />charge was divided by the measured TWA at that discharge to provide the correction factor; all <br />habitat areas for that study site and discharge level were then multiplied by this correction factor. <br />For the 15-mile reach, the relationships were somewhat curvilinear and were best described by <br />natural log transformations (a quadratic polynomial was used to best describe the relationship for <br />Site 4). <br />In cases where the video was filmed one day before or after the day that the habitat was mapped <br />(four of the 11 flights for the 15-mile reach), changes in flow level from one day to the next re- <br />sulted in another source of error. To correct for this, mean discharge on the day of the video flight <br />12
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