Shadow Cliffs Regional Recreation Area is a regional park on the border of Pleasanton, California, and Livermore, California that is part of the East Bay Regional Parks (EBRP) system on Stanley Blvd. The lake was once a gravel pit, but then included a sandy beach with swimming, water slides and it supports fishing and recreational boating. Now the water slides have been taken down and is now planned to be replaced by an Interpretive Center. The park is also a popular picnic ground.
Lake water at Shadow Cliffs Beach is tested for bacterial levels weekly during April through October and twice a month from November to March. In July 2014 EBRP closed the lake to swimmers due to extremely high levels of E. coli bacteria. Testing of water samples demonstrated E. coli at 6,100 colonies per 100 milliliters at the lake's south beach and 1,000 colonies per 100 milliliters at the north beach. California's state standard for daily maximum exposure is 235 colonies per 100 milliliters. Hal MacLean, the district water management supervisor, suggested that a combination of drought conditions, low water levels and accumulated animal feces - probably from geese - had caused the high bacterial content. EBRP planned to treat the lake with an oxidizing agent and "direct geese away from the swim area" in an attempt to lower the levels.