The Whitewater River run from Brothers Wilderness Boundary to Olympic NF Boundary in Washington is a 7.5-mile stretch that offers Class III to IV rapids depending on the streamflow. The ideal streamflow range for this run is between 1,000-2,000 cubic feet per second (cfs), which usually occurs during the spring snowmelt or after heavy rainfall. During low water flows, the rapids become more technical and challenging.

The river has several notable rapids, including "The Notch," "Double Drop," and "The S-Turn." These rapids offer exciting challenges for experienced kayakers and rafters. However, the river also has several dangerous hazards, including strainers and undercut rocks, which require caution and skill to navigate.

This section of the Whitewater River is located within Olympic National Forest and is subject to specific regulations. These regulations include a permit system, which limits the number of daily users. Additionally, the use of motors on the river is prohibited, and all users must practice Leave No Trace principles to minimize their impact on the surrounding environment.

In conclusion, the Whitewater River run from Brothers Wilderness Boundary to Olympic NF Boundary in Washington is a thrilling and challenging run for experienced paddlers. The ideal streamflow range for this run is between 1,000-2,000 cfs, and the river offers Class III to IV rapids. However, users must be aware of the hazards and follow specific regulations to ensure their safety and minimize their impact on the environment.