WSP12636

• File Number: 8282.300
• Description: Colorado River Operations and Accounting - Lower Basin Administrative Procedures
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 8/1/1996
• Author: DOI-BOR
• Title: Description and Assessment of Operations-Maintenance and Sensitive Species of the Lower Colorado River - Volume II - Appendices-Etc - 08-01-96
• Water Supply Pro - Doc Type: Report/Study

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

002531

of Lake Mohave. During the months January through July when Lake Mohave is at a high
elevation, Lake Mohave backs up to Hoover Dam and limits the change in river depth that
the min to max flow change would have during other times of the year. When Lake Mohave
is at its lowest level, usually during November, Lake Mohave has minimal backwater effect
above Willow Beach, 12 miles below Hoover Dam.

Figure E5 shows Lake Mead midnight elevations. It shows that Lake Mead reaches its
maximum elevation in March and its minimum elevation in August as water is released for
downstream use. Inflow to Mead from Glen Canyon Dam upstream reach their peak inflow
in July and their minimum inflow in March which help limit the elevation change in Lake
Mead. The years 1989-1991 show how Lake Mead declines when only the minimum
objective release from Glen Canyon Dam is made and there is high water use in the Lower
basin due to very low rainfall. The data for 1993 show the increase in Mead elevation
caused by flow of the Gila River being delivered to Mexico in lieu of Lake Mead water and
the effect of low water use due to above normal rainfall. 1995 shows the increase in Mead
elevation caused by flow from the Gila River and equalization releases from Glen Canyon
Dam. Equalization occurs when Lake Powell is projected to have more water in storage than
Lake Mead and helps to equalize power production between Hoover and Glen Canyon
powerplants.

Figure E6 shows the range and average Lake Mead midnight elevations by month for years
1982, 1989-1995. The maximum average occurs in March and the minimum average occurs
in August. As noted, the minimum elevation of Lake Mead is at elevation 895 feet, although
downstream water use, mostly by the Central Arizona Project, will be cut back to protect
Southern Nevada intakes at elevation 1050 feet and help limit loss of power generation at
elevation 1083 feet. The maximum elevation is 1229 feet, although the spillway elevation at
1221 feet is maintained if possible by increasing releases as prescribed by flood control
regulations.

Figure E7 shows monthly change in Lake Mead elevation and is computed as end of month
elevation minus previous end of month elevation. The largest monthly elevation increase
occurs in January and averages about +3 feet. The largest elevation monthly decrease
occurs in March and averages about -3 feet.

Historical Davis Dam/Lake Mohave Operations

Figure E8 shows the mean daily releases for Davis Dam. There are 365 daily mean values
plotted for each year. The maximum non-flood year mean daily release is shown to be
22,000 cfs during April 1989 and corresponds with high downstream water use demands.
Hoover uprating appears to have little effect on releases after 1993, but changes in Mohave
target elevations and other operational constraints initiated in 1994 for the Razorback Sucker
backcove rearing program have some effect on releases. The minimum mean daily release of
1,600 cfs occurred during January and February 1993 and was due to operations to limit
flooding in Mexico during the high flows on the Bill Williams River and Gila River.

Appendix E - 3