USACE CWMS - Santa Ana Watershed

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 Santa Ana River Basin is located in Southern California and
has a drainage area totaling 2,460 square miles.. Of the total basin area, 2,255 square miles lie
upstream of Prado Dam, which is the primary flood risk management structure of the Santa Ana River.
The watershed spans mostly within the San Bernardino and Riverside Counties, and portions of Orange
and Los Angeles Counties.
Approximately 23 percent of the Santa Ana basin lies within the rugged San Gabriel and San Bernardino
Mountains, 9 percent within the San Jacinto Mountains, and 5 percent within the Santa Ana Mountains.
Most of the remaining area consists of lower-sloped valleys formed by a series of broad alluvial fan
surfaces which abut the base of the mountain front. Numerous low foothills rise above the alluvial fan
surfaces and include a range of hills north of the San Bernardino; the Crafton Hills east of Redlands; the
Jurupa Mountains north and west of Riverside; the Box Springs Mountains and the Badlands east of
Riverside; and the Chino and Peralta Hills northeast of Anaheim. In general, mountain ranges within the
basin are steep and sharply dissected. Maximum elevations in the Santa Ana basin reach 10,800 feet
NGVD at San Antonio Peak in the San Gabriel Mountains; 11,502 feet NGVD at San Gorgonio Mountain
in the San Bernardino Mountains; and 10,804 feet NGVD at Mount San Jacinto in the San Jacinto
Mountains. San Bernardino Mountains contain the headwaters of the Santa Ana River and two of its
principal tributaries, Bear and Mill Creeks. Lytle Creek, the largest tributary originating in the San Gabriel
Mountains, is in the northwest portion of the watershed. The San Jacinto River has its origin in the San
Jacinto Mountains southeast of Beaumont. The Santa Ana River has an average gradient of about 240
feet/mile in the mountains and about 20 feet/mile near Prado Dam. The average gradients of the principal
tributaries are approximately 700 feet/mile in the mountains and about 30 feet/mile in the valley areas.
The mountainous areas are expected to remain largely undeveloped during the entire project life. The
valley areas below Prado Dam are presently partially urbanized and are expected to approach complete
urbanization by the end of the project life.
The entire Santa Ana River Basin is underlain by a basement complex of crystalline metamorphic and
igneous rocks, which appear on the surface only in the most mountainous parts of the watershed. In the
foothills and valleys, the basement complex is overlain by a series of sandstones and shales.
Unconsolidated alluvial deposits range in depth from a few feet within the mountains to more than 1,000
feet on the alluvial fans in the valleys. The existence of several precipitous mountain scarps along the
upper boundaries of the watershed indicates that the area has been subjected to extensive folding and
faulting. The soils in the mountains, which are derived mainly from metamorphic and igneous rocks, are
shallow, poorly developed, and stony. On the lower slopes of the mountains and foothills, soils are mainly
loams and sandy loams, ranging from less than 1 foot to over 6 feet deep. In the valleys, where soils are
usually more than 6 feet deep, surface soils range from light, sandy alluvium to fine loams and silty clays
with heavier subsoils.
In general, the Santa Ana River Basin has a mild climate with warm, dry summers and cool, wet winters.
Both temperature and precipitation vary considerably with distance from the ocean, elevation, and
topography. At the city of Corona, about 26 miles from the ocean and 710 feet above sea level, the
average temperature is about 63 degrees Fahrenheit, with extremes of 22 degrees Fahrenheit and 118
degrees Fahrenheit recorded. At Squirrel Inn, located in the San Bernardino Mountains at an elevation of
5,700 feet NGVD, the average temperature is about 53 degrees Fahrenheit, with extremes of zero
degrees Fahrenheit and 97 degrees Fahrenheit recorded. Precipitation characteristically occurs in the
form of rainfall, although in the higher elevations some falls as snow. In general, the quantity of
precipitation increases with elevation. The 97-year mean seasonal precipitation for the basin, which
averages about 20 inches, varies from 10 inches south of the city of Riverside to about 45 inches in the
higher mountain areas. Nearly all precipitation occurs during the months of December through March.
Rainless periods of several months during the summer are common.