The Lochsa River is in the northwestern United States, in the mountains of north central Idaho. It is one of two primary tributaries (with the Selway to the south) of the Middle Fork of the Clearwater River in the Clearwater National Forest. Lochsa is a Nez Perce word meaning rough water. The Salish name is Ep Smɫí, "It Has Salmon."

The Lochsa (pronounced "lock-saw") was included by the U.S. Congress in 1968 as part of the National Wild and Scenic Rivers Act. The Lochsa and Selway rivers and their tributaries have no dams, and their flow is unregulated. In late spring (mid-May to mid-June), the Lochsa River is rated as one of the world's best for continuous whitewater.

The main stem of the Lochsa is seventy miles (110 km) in length from its headwaters near Powell Ranger Station in the Bitterroots to Lowell, where the Lochsa joins the Selway River to form the Middle Fork of the Clearwater. Over this distance, the river drops nearly two thousand feet (600 m), from 3,441 feet (1,049 m) above sea level at Powell to 1,453 feet (443 m) at Lowell.

The drainage basin for the Lochsa River system covers 1,180 square miles (3,060 km²) in Idaho County. The river is fed by the melting of the significant snowpack of the Bitterroot Range, among the highest precipitation areas in the state.

The Lochsa River begins at the confluence of Crooked Fork and Colt Killed Creek (also called White Sand Creek) near the Powell Ranger Station in northeastern Idaho and flows 70.1 miles (112.8 km) southwest to the village of Lowell. Running parallel to U.S. Route 12 for its entire length, the river winds through the Clearwater National Forest in the Bitterroot Mountains. Just below Lowell, the Lochsa and the Selway River join to form the Middle Fork Clearwater River.

At its point of formation just west of Elk Summit Road 360, the Lochsa receives a small tributary, Walton Creek, from the left. The creek flows through the Powell adult salmon trap before entering the river. Slightly downstream from the salmon station, the Lochsa passes White Sand Campground on the right. About 1.5 miles (2.4 km) later, it flows by the Powell Ranger Station and heliport and, shortly thereafter, Lochsa Lodge and Powell Campground, all on the right, and receives Cliff Creek and Jay Creek, both from the left.

Between 66 miles (106 km) and 65 miles (105 km) from the mouth, the Lochsa receives Imnamatnoon (Papoose) Creek from the right, then Robin Creek from the left, and passes Whitehouse Campground and Wendover Campground, both on the right, and receives Wendover Creek from the right. Cold Storage Creek enters from the right at about 64 miles (103 km) from the mouth. Eagle Creek enters from the left about 1 mile (1.6 km) later, and Badger Creek from the right 1 mile (1.6 km) further on. About 2 miles (3 km) down from Badger Creek the river receives Waw'aalamnine (Squaw) Creek from the right. Between 60 miles (97 km) and 50 miles (80 km) from the mouth, the river receives Doe Creek from the right, passes under the Jerry Johnson Pack Bridge, receives Burnt Creek from the right and Warm Springs Creek from the left; passes Jerry Johnson Campground on the right; receives Colgate Creek from the left and Mink Creek on the right; passes Colgate Warm Springs and the Colgate Licks on the right, and receives Bear Mountain Creek from the right, Post Office Creek from the right, Lake Creek from the left, and passes under Mocus Point Pack Bridge, in that order.

From about 48 miles (77 km) to 40 miles (64 km) from the mouth, the Lochsa passes Weir Creek Hot Springs and receives Weir Creek and Ginger Creek, both from the right, Mocus Creek from the left, and Ashpile Creek from the right, then Indian Meadows Creek from the left, Indian Grave Creek from the right, passes the White Pine Access Area on the right, receives Eagle Mountain Creek from the left, and passes under Eagle Mountain Pack Bridge, in that order. Over the next 10 miles (16 km), the river receives Skookum Creek from the right, Stanley Creek from the left, and Castle Creek from the right, passes Green Flat Campground and Nine Mile Rest Area, both on the right, receives Bald Mountain Creek from the right, and Dutch Creek and Hard Creek from the left. Just below 30 miles (48 km) from the mouth, the Lochsa receives Pass Creek from the right and over the next 10 miles (16 km) receives No-See-Um Creek, Dipper Creek, and Sherman Creek, all from the right, then Lone Knob Creek from the left, passes Wilderness Gateway Campground on the left and Boulder Flat on the right, receives Boulder Creek from the left, passes the Lochsa Work Center on the right; receives Sardine Creek, Fish Creek, Bee Creek, Eel Creek, and Otter Slide Creek, all from the right, then passes Beaver Flat on the right and receives Big Stew Creek from the left and Snowshoe Creek from the right.