Upper Subbasin Overview
The Upper Subbasin is the largest of the subbasins, occupying two-hundred and six square miles or a little over one-third of the watershed. The watershed area comprises Lake Henshaw on its western boundary and all of the surrounding streams and their drainage areas that flow into the lake (Figure 1). Lake and stream elevations range from 2,727 feet at the spillway on Henshaw Dam (RM 50) to nearly 6,000 feet in the headwaters of a few tributaries on the southwestern portion of the subbasin. The SLR River flows into the lake at approximately 2,730 feet; however, the river’s headwaters approach 5,000 feet in Anza Borrego State Park and exceed 5,000 feet in the headwaters of West Fork SLR River (W.F. SLR River). In general, precipitation increases in the higher elevations of the subbasin. Average yearly rainfall at Henshaw Dam is approximately 26 inches (based on data collected from 1948 to 2006) with higher rainfall totals in the surrounding Palomar, Aguanga, and Hot Springs Mountains. This assessment area is mostly rural, containing only the small community of Warner Springs in the north-central part of the subbasin and Los Coyotes and Santa Isabel tribal members. It is predominantly composed of grasslands and native habitats consisting of mixed sagebrush/chaparral and hardwood forest/woodlands.
Prior to the completion of the Henshaw Dam in 1922, the SLR River at the Henshaw Dam site was a perennially flowing river with minimum monthly summer flows of 1.4 cfs (see Middle Subbasin, Habitat Overview, pp. 11-12). The majority of these flows were most likely the result of the numerous tributaries flowing into the river in the western portion of the subbasin. Historically, the mainstem and most likely the West Fork SLR River, contained large areas of year round flows and supported trout populations of original ocean decent. The first written evidence of native trout in San Diego County comes from a note in an article by Dr. J.G. Cooper from a scientific collection expedition he conducted in 1862 in the Cuyamaca Mountains. He reported that: “trout and stickleback are found no nearer than Warner’s Pass fifteen miles north of San Felipe at the head of the San Luis Rey river” (Cooper 1874). Currently, a self-sustaining, native rainbow trout population is only found in the West Fork SLR River.
To accommodate a popular demand for recreational sport fishing opportunities in inland lakes, warm water game fish, such as largemouth bass, bluegill, green sunfish, black crappie, and catfish were introduced into many lakes and reservoirs in southern California in the late 1940’s. These fish may have been stocked into Lake Henshaw around this time as well. They are now common in the SLR River below the dam and are likely present in other streams within the basin.