The South, West, and Rhode Rivers are brackish water rivers, meaning they are a mix of fresh water (coming from streams, rainwater, and underground springs) and ocean saltwater flowing up the Chesapeake Bay. These brackish rivers, also known as estuaries, play a vital role in the ecosystem, because they are so productive. In other words, estuaries support an abundance of wildlife.
Due to the changing salinity of the water and the changing height of the water due to tides, our rivers are a tough place to live and most of the wildlife have special adaptations to deal with these harsh conditions. However, if you can survive here, like a crab, oyster, or barnacle then your population numbers explode, since there is plenty of food and low levels of competition. It is why the Chesapeake supports so many commercial fisheries. Our rivers have relatively low biodiversity, but high abundance of aquatic life.
This principle applies to plants in our rivers as well. Thus, our rivers historically supported large areas of marsh and underwater grass, which are a vital habitat. Historically, The South, West, and Rhode were important nurseries, where the young of many different species could find protection from larger predators. Unfortunately, development and the resulting stormwater has led to vastly reduced amounts of marsh and underwater grasses in our rivers. They no longer provide the nurseries for wildlife they did in the 1950’s.