The Corps Water Management System (CWMS) includes four interrelated models to assist with water management for the basin:
- GeoHMS (Geospatial Hydrologic Modeling Extension)
- ResSIM (Reservoir System Simulation)
- RAS (River Analysis System)
- FIA (Flood Impact Analysis)
The Mill Creek area is characterized by wide seasonal variations in temperature and wide geographical variations in precipitation. Mill Creek Basin is in the belt of prevailing westerly winds and is largely under the influence of air from the Pacific Ocean. Occasionally, polar outbreaks of cold air spill over the Rocky Mountain barrier resulting in short periods of extremely low temperatures, but generally, winters are characteristically damp and foggy.
Mean annual precipitation for climate stations in the basin ranges from 17.8 inches at Walla Walla WB City, El. 948, in the lower portion of the basin, to 41.9 inches at Walla Walla 13 ESE, El. 2,400 (Refer to Tables 4-3 and 4-4). It is probable that at elevations above 5,000 feet, mean annual precipitation exceeds 50 inches. At Walla Walla, approximately 10 percent of the normal annual precipitation falls as snow; at higher elevations, this percentage is increased considerably, becoming approximately 40 percent at the 5,000-foot level. The normal annual precipitation for the basin is estimated to range from 35 to 40 inches above the project.
The general pattern of streamflow for Mill Creek consists of moderate to high flows from November through June and low flows from July through October. During years of low autumn precipitation and below normal winter temperatures, the period of low 4 - 9 flows may extend as late as February.
Major floods may be caused from any one of the following conditions: (1) intensive rainstorms, (2) a combination of rainfall and snowmelt, or (3) summer "cloudburst" thunderstorms. The winter flood period generally extends from December through February. Winter floods are flash-type floods that are relatively short in duration with peak discharges occurring in December through February. Historical floods of damaging magnitudes on Mill Creek have generally occurred in the winter and have been caused primarily by runoff from intense rainfall on snow with frozen ground or ground with a high soil moisture content. The spring snowmelt flood period generally extends from about the first of March through May. Peak discharges from snowmelt only runoff, rarely results in damaging stages. For the 1942-2005 period of record, the maximum mean daily discharge was 1,970 cfs on 23 December 1964 and the minimum mean daily discharge generally reaches zero in August. The largest historical flood outside of this period of record occurred on 1 April 1931 with an estimated peak discharge of 6,000 cfs.