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Floods ' Measuring changes in channel form during flood passage is more difficult than observing the resultant morphology at low flow. Much of our understanding of channel response to flood passage comes from measurements made at USGS cableways. Measurements made at these sites show that sand-bedded alluvial rivers typically scour on the ascending limb and fill on the descending limb of floods, although Leopold et al. [1964] noted that cableway cross sections may be biased by their siting in pool sections of rivers. ' Leopold et al. [ 1964] used the response of the Colorado River at Lees Ferry to the passage of the 1956 spring flood as an example of the typical response of sand-bedded rivers at gaged cross sections. The bed scoured approximately 3 m on the ascending limb of the ' hydrograph. The maximum depth of scour and the flood peak coincided. The bed subsequently filled on the descending limb to about the pre-flood level [Leopold et al., 1964]. Additional work on the Rio Grande del Ranchos, a tributary of the Rio Grande in New Mexico, and Baldwin Creek and Popo Agie River in Wyoming found net scour at flood peak over long reaches containing both pools and riffles [Leopold et al., 1964]. Colby [1964] found that single cross- sectionmeasurements, such as those made at gaging stations, could not be used to characterize ' the behavior of a stream reach, however. Cross sections within a single reach could be characterized as either filling or scouring cross sections, with fill or scour occurring on both the ascending and descending limb of the flood. In addition, Colby [1964] found that streams typically adjusted to changing discharge by changes in the water surface elevation rather than by scour or fill of the stream bed. Consequently, cross-section studies to characterize river response to flood passage must include cross sections spaced throughout a representative reach. Discharge measurement records from the discontinued USGS gaging station near Ouray (station number 09307000) surveys were analyzed by Schmidt [1994]. His analysis for the gage 1 1 1 m? E- vi ~ oc ~Q ~a z~ zq oa Fa ¢z ~a ,.a u~ 5 4 0 -~ -2 -3 -4 •5 annual spring . / floods`. water surface i~ ~ ~ ~1 bed elevation 1 1000 2000 3000 4000 5000 NUMBER OF DAYS FOLLOWING JAIVLIARY 1, 1951 Figure 3. Water surface and thalweg elevation for the US Geological Survey stream gage Green River near Ouray, Utah for the period 1951 to 1965. The thalweg at the gage scoured on the ascending limb of the flood hydrograph, but quickly returned to its pre-flood elevation. This demonstrates the dynamic nature of the Green River, even at a "stable" site deemed suitable for a USGS gage. A-7