Watersheds are drainage basins where runoff from precipitation or springs is channeled. The extent of the watershed depends on the area and topography of the land draining to the same discharge point (USGS 2013b). Inland watersheds are areas where instream flow drains into discharge points where it either pools in lakes and ponds, and evaporation occurs, or it infiltrates the soil and becomes incorporated into ground water (USGS 2013b). Coastal watersheds may include estuaries where rivers and streams flow into the oceans and mixing occurs. Again, this water is referred to as freshwater inflows. The sources of freshwater inflows begin mostly upstream and include rainfall, river flow, runoff, groundwater, and return flows. Rainfall is a freshwater source that can occur directly on the estuary. The sources of freshwater influence the amount, timing, frequency, and duration of the freshwater inflows into the estuary (Montagna et al. 2013).