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 Los Angeles County Drainage Area (LACDA) is located in
Southern California and has a drainage area totaling 1,459 square miles. The watershed spans mostly
within the Los Angeles County, and extends to portions of San Bernardino and Orange Counties. The
watershed is abutted on the east by the Santa Ana River Basin, on the north by the Antelope Valley and
Santa Clara River Basins, and on the west by the Calleguas Creek Basin.
Elevations in the San Gabriel and Santa Susana Mountains, which form the northern boundary of the
watershed, vary from over 9000 feet in the east to 3000 feet in the west. The Santa Monica Mountains,
Montebello Hills, and Puente Hills separate the San Fernando and San Gabriel valleys from the coastal
plain, and their elevations range from 500 to 1500 feet.
Principal streams in LACDA are the Los Angeles River, (including the Rio Hondo above Whittier Narrows
Dam and its tributaries), and the San Gabriel River which have drainage areas of 824 and 635 square
miles at the mouth, respectively. The principal tributaries of the Los Angeles River include: Pacoima and
Tujunga Washes, both of which drain portions of the Santa Susana Mountains and the San Fernando
Valley; the Arroyo Seco, which starts in the San Gabriel Mountains and then heads south to the Los
Angeles River; and Compton Creek, which drains part of the coastal plain. The main channel of the Los
Angeles River is approximately 50 miles long and its tributaries have an aggregate length of about 225
miles. During periods of high runoff, the lower Rio Hondo Diversion Channel brings water from the San
Gabriel River system to the Los Angeles River, which could effectively increase the contributing drainage
area to the Los Angeles River during periods of high runoff. Principal tributaries of the San Gabriel River
include: Big and Little Dalton Wash, San Dimas Wash, and Walnut Creek, all of which drain portions of
the San Gabriel mountains and the San Gabriel Valley; San Jose Creek which drains the San Gabriel
Valley; and Brea Creek, Fullerton Creek, and Carbon Creek, which drain the coastal plain and are
tributary to the San Gabriel River via Coyote Creek. The San Gabriel River is approximately 58 miles long
and its tributaries in aggregate length total about 76 miles. The Rio Hondo, which is tributary to the Los
Angeles River, also connects with the San Gabriel River within in the Whittier Narrows Flood Control
Basin. While the Rio Hondo diverts much of the San Gabriel River runoff to the Los Angeles River, it also
drains the adjacent area to the north and northwest. The tributary area of the Rio Hondo is 137 square
miles or about 9 percent of the LACDA basin. Its length is approximately 20 miles and the aggregate
length of its tributaries is about 60 miles. The principal tributaries are Sawpit Wash, Santa Anita Wash,
Arcadia Wash, Eaton Wash, Rubio Wash, and Alhambra Wash. Stream slopes range from very steep in
the mountains, with slopes over 200 feet per mile common, to approximately 3 feet per mile in the coastal
plain.
In the mountains, runoff concentrates quickly from the steep slopes; hydrographs show that the stream
flow increases rapidly in response to effective rainfall. High rainfall rates, in combination with the effects of
shallow surface soils, impervious bedrock, fan-shaped stream systems, steep gradients, and occasional
denudation of the area by fire, result in intense debris-laden floods. These flood and debris-laden flows
are regulated at existing dams and debris basins.