River herring is the general name for two species of fish commonly found in the Bay, the alewife and the blueback herring. They are similar to one another, differing only slightly in appearance and behavior. The blueback herring is thinner and has a distinctive blue black back, while alewives are thicker and more greenish black. This distinction is most apparent in freshly caught fish. River herring are also similar to a group of larger herring called shad.

River herring are thick around the head and abdomen, narrowing to the tail, with heavy sharp serrations along the underside, hence the name "sawbelly." Its scales are large and slough off easily when handled. It has a distinctive dusky grey spot just behind the margin of the gill cover. This fish has large eyes, a lower jaw that projects beyond the upper jaw, and a toothless mouth.

River herring are anadromous fish that spend their adult lives in the ocean and return each year to spawn in the freshwater streams where they hatched. Herring can often be found in schools of thousands, congregating near the area where they hatched. River herring suffer high rates of natural mortality, with fewer than one percent of all eggs surviving the harsh migration to salt water as juveniles. Most adult herring die during or after spawning migrations.

The river herring feeds primarily on plankton, including copepods, amphipods, shrimp, and fish eggs. It is preyed upon by striped bass, bluefish, gulls, terns, and other coastal birds.

Caught mainly in the mouths of the streams and rivers during spawning, both species of river herring are commercially important as a food source as well as bait. The river herring's habitat range has been largely restricted due to overfishing, pollution created by manufacturing wastes, and erection of dams. These dams, many of which are still in existance today, block rivers vital to spawning migrations and greatly reduce the ability of the population to be sustained.

State and nonprofit environmental groups are attempting to reestablish alewife stocks by building fish ladders around dams, cleaning up rivers, and introducing spawning adults from other locations to enhance existing Rhode Island alewife populations.