Middle Subbasin Overview

The Middle Subbasin is the smallest of the subbasins, occupying twenty-six square miles. The watershed area is bracketed by the Escondido Canal diversion dam on the western side (the diversion is just downstream of the downstream subbasin boundary) and the Henshaw Dam on the eastern end (Figure 2). This subbasin includes the SLR River from RM 41, just upstream of the diversion, and all of its tributaries upstream to RM 50, Henshaw Dam. Stream elevations range from 1,700 feet in the western portion of the SLR River to approximately 5,000 feet in the headwaters of tributaries draining the eastern portion of Palomar Mountain. The elevation of the river at the base of the dam is approximately 2,700 feet. 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) (http://www.wrcc.dri.edu/).

The Middle Subbasin assessment area is rural with no concentrated housing communities. It is predominantly composed of native habitats consisting of mixed sagebrush/chaparral in the lower elevations and hardwood forest/woodlands in the higher elevations. Ownership is split almost evenly between private ownership, La Jolla Band of Luiseño Indians Reservation, and US Forest Service (Cleveland National Forest). The western portion of the subbasin was greatly impacted by the 2007 Poomacha Fire as over 9,000 acres (90%) of the La Jolla Indian Reservation was burned, including the loss of 55 homes and displacement of 180 tribal members (http://www.lajollaindians.com/).

Prior to the completion of Henshaw Dam, the SLR River at the dam site was a perennially flowing river. According to historic USGS stream gauge data recorded from 1912 to 1922 at the present dam site, the river maintained minimum monthly summer flows of 1.4 cfs, while minimum monthly winter and spring flows averaged above 8 cfs (see Figure 5, p.11). Former California CDFG biologist, Gary Shaw, speculated that the river below the dam site “supported a minimum trout fishery” (Jones and Stokes Associates, Inc. 1976). Moreover, trout were first documented in the Upper Subbasin, headwaters of the SLR River as early as 1862 (Cooper 1874). This written documentation was well before the introduction of hatchery raised fish, indicating the movement of steelhead trout through the Middle Subbasin. Currently, the Escondido Canal diversion dam (RM 40) prevents passage into the Middle Subbasin. Additionally, a waterfall in the SLR River canyon (RM 39.5) and multiple partial fish passage barriers would also restrict steelhead from accessing the potentially suitable spawning and rearing habitat found in the Middle Subbasin.

Fish passage into the subbasin is only one of several problematic issues concerning the possibility of steelhead/trout successfully utilizing the Middle Subbasin. Water releases from the Henshaw Dam may not coincide with the freshwater life cycle stages of steelhead trout. The amount and timing of water releases by Henshaw Dam are based on water right agreements with the La Jolla Indians and the Rincon Band of Mission Indians, precipitation totals, as well as water supply needs of the City of Escondido. Altered hydraulic processes, poor to moderate instream habitat conditions, and numerous, exotic, predatory game fish all contribute to less than ideal conditions for steelhead trout production.

Rainbow trout were once stocked in the SLR River just downstream of Lake Henshaw and within the La Jolla Indian Reservation to accommodate a popular demand for recreational sport fishing opportunities in the region. The stocking of trout has ceased in recent years due to drought-like conditions and because of concerns of stocked rainbow trout competition and possible predation of the federally listed arroyo toad, Bufo californicus. No progeny from these trout were observed during CDFG 2007 electro-fishing sampling or while performing the habitat inventory in the upper five miles of the SLR River.

A view of the Upper Middle Subbasin as seen from Henshaw Dam, facing west.
Picture taken in September 2007.