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29 3.0 HYDROLOGIC ANALYSIS 3,1 BASIN FIELD TRIPS Prior to planning a field trip to the Colorado River basin, previous Reclamation flood studies ,were reviewed for other dams in the basin and for similar basins, such as Roosevelt Dam in Arizona. Twenty seven studies were reviewed, and a compilation of lag-equation coefficients and loss rates was made. All of the recent studies were in the mountainous areas above Glen Canyon Dam. The areas which lacked hydrologic information and recent field investigations were the lower elevation, desert and foothill areas, plus nearly all of the Little Colorado River basin in northern Arizona. The field trips concentrated on collecting information in these areas, while relying on data from past studies for the necessary hydrologic parameters in the upper reaches of the basin. Field trips were made through the lower basin (Hoover Dam to Glen Canyon Dam) and the upper basin (above Glen Canyon Dam) in June, 1987. Personnel from the Lower Colorado Regional Office and Denver Office performed a reconnaissance of the lower part of the basin on June 1-3, 1987. They visited areas adjacent to Lake Mead (north and south sides), Muddy Creek, Meadow Valley Wash, Santa Clara River, Virgin River, Kanab Creek, Kaibab Creek, northern tributaries to the Little Colorado River, Zuni River, and the Little Colorado River. Personnel from the Upper Colorado Regional Office and Denver Office performed a reconnaissance of portions of the upper and lower basin, June 1-10, 1987. They visited the areas tributary to Lake Powell including the Paria River, Escalante River, Fremont River, Dirty Devil River, San Rafael River, lower Green River, and San Juan River. The field trips were made to become familiar with the subbasins: to observe soil and geologic conditions for use in estimating loss or infiltration rates: to observe land use, vegetation type and cover, and basin roughness and steepness; and to observe evidence of previous high flows. Estimates of lag coefficients ("C" values) and loss rates, for use in deriving the probable maximum floods, were made for all of the subbasins visited. In general, the lower basin near Lake Mead and the northside tributaries to Lake Mead are areas of low infiltration rates and are subject to flash flooding. The other areas and tributaries, especially Kanab Creek, Kaibab Creek, and most of the Little Colorado River basin had somewhat higher loss rates. (Loss rates are a measure of the precipitation lost to infiltration, , evaporation, transpiration, and absorption, and also to minor detention storage in the basin.) In these areas, the vegetative cover was heavier and the loss rates appeared to increase with elevation rise. Most of the Little Colorado River basin showed very little evidence of flash flooding or stream channel development. In the upper basin, those areas tributary to Lake Powell were very desert-like and exhibited signs of flash flooding. The loss rates appeared quite low, and the vegetative cover was very sparse. Some