The following is quoted from an explanatory sign in the nature preserve:

"The San Gabriel Alluvial

The Santa Fe Dam nature area is a biologically complex habitat unique to the Los Angeles basin. It consists of a special community of plants and animals living on the broad alluvial fan of the San Gabriel River at the base of the San Gabriel mountain range; it is located about four miles downstream from the mouth of the San Gabriel Canyon, and about 30 miles upstream from the ocean.

For millions of years the area has been subject to periodic flood disturbance during winter rains. Water has carried mountain debris, boulders, small rocks, and sand down into the valley below. At the base of the fan, where the water pours swiftly out of the narrow canyon mouth, the area tends to be rocky and very coarse in texture; near the outer edges, the texture is finer, and sandy. As water loses it velocity, its capacity for carrying debris decreases. The fan is known to be over 1000 feet thick in some parts and provides materials for a large gravel sales industry in the area. There are about a half dozen alluvial fans of comparable size in the Los Angeles basin, including the Big Tujunga Wash and the Santa Anita Canyon to the West, the San Antonian Canyon, the Cajon Wash and the Santa Ana Wash to the East.

The Santa Fe Dam nature area's 1000 acres help preserve the last vestiges of an extremely complex vegetative community that has now been nearly destroyed by human development. Here on the alluvial fan a plant community has grown up which is especially well adjusted to the rigorous conditions of regular flooding, cool moist winters, and hot dry summers. The meandering channels of the San Gabriel River can be described as a "highly disturbed wash community" in which vegetation has little opportunity to become established due to flooding. The predominant plants here are annual grasses and small shrubs with strong "pioneering" abilities.

Along the banks of the wash area, where flooding has not been so devastating, a Riparian Woodland community has sprung up, characterized by stands of willows and sycamore trees which require the presence of a year around water supply.

Most of the plants in the Santa Fe Dam area are a blend of two distinct biological communities, the Chaparall and the Coastal Sage. Chaparall is characterized generally by low, dense, evergreen shrubs with stiff, hard, thick leaves. The thick leaves help conserve water during the long dry season. There are very few trees and very few herbs in a Chaparall community. The dominant species are Chamise or Greasewood, California Holly of Toyon, Scrub Oak, Yucca, California Lilac or Ceanothus, Sugar Bush and Yerba Santa. The Coastal Sage plant community occurs on the coastal slopes of Southern California mountains in dry, gravelly, or rocky slopes below 3000 feet. The plants rarely grow over 6 feet tall. The dominant species are, of course, the Sages, but various species of Cactus are plentiful, as well as Lemonadeberry, California Buckwheat, and Scalebroom.

Some common mammals in the area include the Coyote, the Pacific California Ground Squirrel, the Desert Cottontail Rabbit, the Kangaroo Rat, and the Wood Rat, sometimes also called the Pack Rat, which is famous for collecting odd debris for it's nests.

In addition to numerous migratory bird species, the area supports many permanent bird residents, including the California Quail, the Ring Necked Pheasant, the Killdeer, the Mourning Dove, and several kinds of hummingbirds, hawks, owls, woodpeckers, wrens, goldfinches and sparrows.

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