Its two main branches start in southwestern Massachusetts. The West Branch starts in Becket, Massachusetts and flows southeasterly to Otis, Massachusetts. A 14-mile (23 km) portion of the western branch has been designated a National Wild and Scenic River. The eastern branch is now mostly covered by the Barkhamsted Reservoir. The two branches join in New Hartford, Connecticut. The upper reaches of the river flow mostly southward, but the river turns northward in Farmington, Connecticut and then runs mostly north and east until it flows into the Connecticut River near the Loomis Chaffee School in Windsor, Connecticut.
A second whitewater section is found in Tariffville, Connecticut, one mile (1.6 km) of technical Class 3 water which is runnable all year round. The river is normally paddled at levels between 1.5 and 2.75 feet (45-75 cm) on the internet gauge ; above 2.5 feet (75 cm) it becomes significantly heavier and more dangerous. This section includes the famous T-ville Hole, where kayakers can practice hole surfing and freestyle moves above a flat pool. Below the Hole is a broken dam, where the river funnels through an abrupt four foot drop into a large wave. This area is popular with swimmers in summer, and it is risky due to heavy currents and undercut rocks. There have been at least three fatal drownings in the Tariffville Gorge section, primarily people who were not properly prepared or trained for the heavy rapids and pinning obstacles in the gorge. Paddlers without helmets, lifejackets and Class 3 whitewater skills should end downriver trips at Tariffville Park, just above the start of the gorge.
Other whitewater areas include Satan's Kingdom in Riverton, Connecticut, which is popular with tubers, and the Collinsville section in Collinsville and Unionville, Connecticut, which is about four miles of Class 2 training water with a bicycle and pedestrian path on the right side of the river. Entrance to the park is free. A service will also pick tubers up and drop them off at certain points.
The west branch of the river includes two hydroelectric dams in West Hartland and Colebrook, run by Connecticut's Metropolitan District Commission.
The largest dam on the east branch is the Saville Dam, which impounds the Barkhamsted Reservoir.
The Rainbow Dam, a 68-foot dam with a hydroelectric generator and a fish ladder, dams the river at Windsor, a few miles before the river flows into the Connecticut River.
A number of other dams have been built on the river since European settlement, usually to power mills and other industry. A few, such as in Collinsville, are still mostly intact.
Water released from or flowing over the Otis Reservoir dam enters the Farmington River just North of Reservoir Road in Otis, Ma. Significant quantities of water are released during the fall in order to drop the reservoir water level for the winter.
In both October 2005 and May 2006, heavy rains deluged the Farmington River Valley and the Farmington rose to flood numerous forests and fields near the towns of Simsbury and East Granby. The river was so high in October 2005, that the river flowed over an old broken dam on the East Granby-Tariffville border.