Channel adjustments from instream mining: San Luis Rey River, San Diego County, California
Michael Sandecki, Catherine Crossett Avila, 1997. "Channel adjustments from instream mining: San Luis Rey River, San Diego County, California", Storm-Induced Geologic Hazards, Robert A. Larson, James E. Slosson
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The San Luis Rey River comprises a 1,450-km2 (560-mi2) watershed in northern San Diego County, California. Construction aggregate has been mined along a 22.5-km (14-mi) reach of the river. Cumulative extraction volumes of eight operators peaked in the late 1980s, with few restrictions or coordinated oversight by local, state, or federal agencies. The river channel was deepened and widened as sand was removed at rates far in excess of natural replenishment. The 1992-1993 floods caused headward erosion of the mined pit boundaries, interruption of sediment transport continuity, and downstream scour. Lowering the base level in the mined portions triggered rapid erosional adjustments in nonmined portions of the river, affecting infrastructure, adjacent property, and wildlife habitat. During the 1992-1993 storms, the riverbed degraded 2.4 to 3.7 m (8 to 12 ft) under the old Route 395 bridge, causing structural instability that closed the bridge to traffic and necessitated a $4.5 million bridge replacement project. The Route 76 bridge over a tributary to the San Luis Rey River failed as the tributary headcut upstream, lowering the bed in the mainstem. The exposure of aqueduct crossings, sewage lines, natural gas conduits, and bridge foundations prompted a comprehensive evaluation of instream mining activity, initiated by the San Diego County Water Authority in 1990. Concurrently, the Environmental Protection Agency authorized funding a watershed management plan to preserve or replace habitat critical to rare, threatened, and endangered species. The lessons learned from the San Luis Rey River include: (1) the cumulative impacts of sand removal should be quantified, and potential offsite impact areas identified; (2) the effects of bed lowering on infrastructure can be quantified and used to limit mining depths and locations; and (3) the loss of riparian habitat can be minimized by identifying affected areas, preserving critical areas, and promptly implementing aquatic habitat and wildlife enhancement programs to restore impacted areas.