The Los Peñasquitos Hydrologic Unit is comprised of the Los Penasquitos Creek watershed, several coastal tributaries, and the Mission Bay watershed.  These watersheds drain a highly urbanized region located almost entirely west of Interstate 15 in coastal San Diego County.  Collectively and individually, they support a variety of water supply, economic, recreational, and habitat-related beneficial uses.  The major receiving waters, Los Peñasquitos Lagoon and Mission Bay, are both fragile systems that support diverse native fauna and flora.  Both water bodies are especially sensitive to the effects of pollutants due to restricted or intermittent tidal flushing. Combined the Los Peñasquitos Hydrologic Unit and the Mission Bay Hydrologic Unit drain 180 square miles.