This study aims to measure the historical extent of California estuaries over the past 140 years using geographic information systems (GIS) to analyze historical maps and compare them to current imagery. The researcher has measured three area metrics (area within the 20 foot contour, apparent water body, and wettable area) for 50 estuaries from Arroyo Hondo to Pismo Creek Lagoon. These measurements were taken from maps and topographic sheets dating back to 1873 and compared to modern imagery. The goal is to enhance understanding of how estuary extent has changed over time due to past management and infrastructure in order to improve restoration strategies. Future work will continue these measurements along more of the California coast and classify estuaries according to