The Portneuf River flows 124 miles through southeastern Idaho, originating in western Caribou County along the eastern flank of the Portneuf Range, approximately 25 miles east of Pocatello. The river initially flows south before wrapping around the southern end of the range, then turns north to carve through the valley between the Portneuf Range to the east and the Bannock Range to the west. It passes through downtown Pocatello before emptying into the Snake River at the southeast corner of American Falls Reservoir.

This river system offers remarkably varied paddling experiences, from family-friendly urban floats to serious Class IV whitewater challenges. The Pocatello Water Trail provides the most accessible paddling, with a Class I float through the heart of the city featuring multiple access points at Rainey Creek, Taysom Rotary Park, Cheyenne Bridge, and Portneuf Road. This section moves at a leisurely pace downstream of Taysom Rotary Park and is popular for tubing, kayaking, and canoeing from May through October.

Near Lava Hot Springs, the river transforms into a rapid-filled playground. The 6.1-mile stretch from Sportsmen Campground to the PVC diversion offers Class II-III whitewater through scenic terrain, while the Johnny Creek to I-86 section presents a formidable 15.6-mile Class IV run that demands serious whitewater skills and sees relatively light traffic. The Diversion to Marsh Creek section covers 14.3 miles through Bannock County, though specific details about this stretch remain limited.

The river's character has been significantly shaped by human intervention. Following severe floods in the early 1960s, the Army Corps of Engineers constructed a concrete channel through western Pocatello in 1965, fundamentally altering the river's natural processes through the urban corridor.

Brown trout, cutthroat trout, and rainbow trout inhabit the river system, with special regulations protecting cutthroat populations in certain sections. From the East Main Street bridge in Lava Hot Springs upstream to Chesterfield Reservoir, catch-and-release regulations with barbless hooks apply from December through the Friday before Memorial Day weekend.

Paddlers must navigate complex access restrictions, as much of the adjacent land is privately owned. Entry and exit are permitted only from public land or at public rights-of-way near bridges. Portions flowing through the Fort Hall Reservation fall under tribal jurisdiction. Downed trees and submerged logs present the most significant hazards, particularly in intermediate and advanced sections, where paddlers should expect to portage obstacles. The section through downtown Lava Hot Springs is closed to fishing year-round to accommodate swimmers and tubers.