This largest undeveloped expanse of habitat in western Riverside Co. features a large reservoir (Lake Mathews) surrounded by annual grassland and arid, Riversidean Coastal Sage Scrub that extends over hilly terrain about 10 miles south to Interstate 15. Much of the land has been designated the Estelle Mountain Lake Mathews Reserve with parcels owned by the Metropolitan Water District (MWD), the Riverside County Habitat Conservation Authority, DFG and BLM. Day-to-day management is coordinated by the non-profit Center for Natural Lands Management. At 13,000 acres, this reserve encompasses most of the open space immediately surrounding Lake Mathews and the hills to the south. While the reserve south of the lake is fenced and tightly monitored, an annex of the reserve west and northwest of the lake is afforded minimal protection. Arlington Mountain and Estelle Mountain are floristically similar with the addition of large tracts of chamise chaparral and even Western Juniper-dotted scrub on the latter, a unique habitat type here restricted to the foothills of Peninsula Ranges. Temescal Wash drains the southern flank of Estelle Mountain, and supports lush riparian woodland with oaks, willows and sycamores.

Ornithological Summary

This avifauna of this IBA is notable for supporting one of the largest populations of California Gnatcatchers in Riverside Co., as well as for its high number and diversity of wintering raptors. Lake Mathews has become an important wintering area for Bald Eagle in southern California. Temescal Wash, on the southern end of the IBA, supports an intact riparian bird community with both foothill and lowland elements.

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Conservation Issues

Originally designated to provide Critical Habitat (per USFWS) for the Stephen's Kangaroo-Rat, this area has emerged as one of the key areas for conservation attention under the Western Riverside County Multi-species Habitat Conservation Plan. As such, its protection under this plan should be carefully monitored. The ongoing conversion of high-quality coastal sage scrub to exotic, annual grassland (largely through arson) represents the foremost threat (R. Baxter, Center for Natural Lands Management, pers. comm.). Away from patrolled areas within the Estelle Mountain - Lake Mathews Reserve, major human disturbance (including OHV use, arson, illegal hunting, dumping) threatens the remaining patches of Riversidean Coastal Sage Scrub (especially vic. Arlington Mountain). Urban sprawl continues to claim open space in the region - the development on the northwest slope of Arlington Mtn. (mid-1990s) is now infamous for knowingly causing the extirpation of one of the last pairs of Golden Eagle in western Riverside Co. Educational outreach to the Metropolitan Water District could greatly enhance riparian habitat there - the willow forest surrounding Lake Mathews has long been cleared for the specific purpose of eliminating habitat for sensitive bird species (L. LaPre, pers. comm.). Although a large area of habitat is protected as open space, habitat on the southwestern flank of Estelle Mtn. and adjacent Temescal Wash is seeing the construction of scattered "dream homes", housing tracts and most recently, a massive mining operation.

Ownership

Much of the land has been designated the Estelle Mountain ? Lake Mathews Reserve with parcels owned by the Metropolitan Water District (MWD), the Riverside County Habitat Conservation Authority, DFG and BLM. Day-to-day management is coordinated by the non-profit Center for Natural Lands Management.

Habitat

This largest undeveloped expanse of habitat in western Riverside Co. features a large reservoir (Lake Mathews) surrounded by annual grassland and arid, Riversidean Coastal Sage Scrub. Arlington Mountain and Estelle Mountain are floristically similar with the addition of large tracts of chamise chaparral and even Western Juniper-dotted scrub on the latter, a unique habitat type here restricted to the foothills of Peninsula Ranges. Temescal Wash drains the southern flank of Estelle Mountain, and supports lush riparian woodland with oaks, willows and sycamores.

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