Beels are shallow waterbodies with rich macrophytic vegetation. The recent satellite survey undertaken by the Assam Remote Sensing Application Centre, shows that there are as many as 3,536 wetlands in the state, occupying an area of 1,030,76 ha constituting 1.31 per cent of the total geographical area of the state. The largest three beels are Son in Karimganj district (3,010 ha), Nandani-Sonai in Morigaon district (1,350 ha) and Batha in Darrang district (1,230 ha). Most wetlands are linked to the Brahmaputra or her numerous tributaries by natural canals and waterways. They also serve as sinks for excess water spilling over from the rivers during the monsoons, and act as receptacles for the runoff from the surrounding hills and raised lands. The river spillage and the surface water runoff are important inputs to the beels, not only recharging their water, but injecting valuable dissolved nutrients crucial to their productivity. During the monsoons, the beels overflow to submerge the marshy vegetation. With the recession of floods, the water level decreases; and, finally, during the lean months from November to April, the water recedes considerably, exposing sizeable pockets of marshy vegetation, which prove excellent feeding grounds for visiting waterfowls.
The beels are also excellent breeding and spawning grounds for fish and other aquatic fauna. During the monsoons, the incoming water brings gravid females of the riverine fish varieties, both large and small, looking for suitable spawning grounds. Spawning over, the fish take up residence for the rest of the season in the beels. As the eggs hatch, the fry find food and shelter in the nooks and crevices among the vegetation, feeding and growing in waters rich in organic matter. As the flood waters recede, the fingerlings hatched in the beel waters await the right moment to migrate back to the rivers.