Prospect Island is a small island in the San Joaquin River delta, in California. It is part of Solano County, and managed by Reclamation District 1667. Its coordinates are 38°14′47″N 121°39′56″W / 38.2463030°N 121.6655110°W. It appears on a 1952 USGS map; by 1978, survey maps show it cut diagonally by the Sacramento River Deep Water Ship Ch…Prospect Island is a small island in the San Joaquin River delta, in California. It is part of Solano County, and managed by Reclamation District 1667. Its coordinates are 38°14′47″N 121°39′56″W / 38.2463030°N 121.6655110°W. It appears on a 1952 USGS map; by 1978, survey maps show it cut diagonally by the Sacramento River Deep Water Ship Channel.