The topography of Puerto Rico and the U.S. Virgin Islands (USVI) is characterized by steep terrain and short distances to the sea. This means that freshwater runs off the islands quickly, coming into contact with seawater in coastal estuaries. The physical characteristics of estuaries change as the tides rise and fall, creating a wide range of habitats that support diverse plants and wildlife, including economically and culturally important native species such as cetí and land crabs, as well as game fishes such as snook and tarpon. These ecosystems are already heavily threatened by human activities such as urbanization, increased sedimentation, and pollution. Changing climate conditions, such as more frequent and severe drought, pose an additional stressor. Because rivers in Puerto Rico and temporary streams (known locally as “ghuts”) in the USVI  feed the coastal estuaries of the U.S. Caribbean, changes to streamflow can impact estuaries and the wildlife they support. For example, during prolonged periods of low flow, withdrawals from the Espiritu Santo River, which feeds into the Espiritu Santo estuary in northeast Puerto Rico, can reach 100% of instream flow and the river can run dry (Benstead et al., 1999). This reduction in the amount of freshwater entering the estuary can increase salinity levels, altering habitat conditions and leading to declines in the richness and abundance of freshwater species (Smith et al., 2008).