The Big Lost River is a significant waterway in central Idaho, originating in the Pioneer Mountains and flowing southeast through Custer County. The river's main stem is formed by the confluence of its East Fork and West Fork near Copper Basin. The East Fork, approximately 27.9 miles long, is a major tributary that offers some of the most exciting whitewater opportunities in the system.

Paddling enthusiasts are drawn to the East Fork Big Lost River, particularly the upper 15-mile section from near Copper Basin Airport to the confluence with the West Fork. This stretch offers challenging Class III-IV rapids, making it suitable for experienced kayakers and rafters. The ideal flow range for this section is 300-800 cfs, providing thrilling whitewater without becoming overly hazardous. The difficulty of the river varies, with the upper sections being more technical and demanding. Paddlers should have solid Class III-IV skills and be comfortable with remote wilderness conditions. The lower sections of the main Big Lost River tend to be more suitable for fishing and scenic floating.

Spring runoff, typically peaking in May and June, offers the best whitewater conditions. However, the river can be paddled from late spring through early fall, depending on water levels. It's crucial to check current flow data before planning a trip. The river flows through a diverse landscape of dry, partly wooded mountain valleys and rugged Idaho terrain. Wildlife such as elk, deer, and various bird species can often be spotted along the banks. The surrounding area is known for its beautiful wildflowers in spring and early summer.

Key access points include the put-in near Copper Basin Airport on Burma Road for the East Fork's upper section, and various take-outs along Forest Road 135. The nearest towns are Mackay and Arco, which serve as bases for river expeditions. The Big Lost River and its tributaries have played a significant role in the region's mining and agricultural history. Today, the river is an important resource for irrigation and supports a thriving recreational economy.

Safety considerations include the river's remote nature, potential for rapid water level changes, and the presence of logjams and strainers. Paddlers should be self-sufficient and prepared for wilderness emergencies. Local regulations may apply, particularly regarding fishing and camping along the river corridor. While most trips on the East Fork are single-day adventures, multi-day expeditions are possible on the main stem of the Big Lost River, allowing for a more immersive experience in the Idaho backcountry. These longer trips require careful planning and a good understanding of the river's flow patterns and access points.