Depending on the underlying geology and surrounding topography, there are many kinds of stream and river habitats. Some in rocky, higher elevation areas have steep gradients and the waters are cold, highly oxygenated and fast-flowing. These are the conditions needed by cold-water fish like brook trout and they're also the haunt of some of the state's rarest dragonflies. These streams are in the upper reaches of a watershed.

They eventually join larger, slower-moving rivers like the Connecticut River, which is the largest river in New England. It begins near the Canadian border and travels 410 miles to Long Island Sound. Unlike some of the shady streams up in the mountains, the Connecticut River is warmer and its bed is made of sands and silts. Thanks to improvements in water quality, the Connecticut River and its tributaries provide habitat for 90 different rare species, including several species that are protected under the Federal Endangered Species Act.