Each winter the District removes accumulations of woody debris from the Boise River to help maintain a clear channel to reduce the risk of flooding during high river flows. Loose debris can get caught up on bridges or other channel obstructions during higher flows and cause localized flooding damage. Once an obstruction causes the water to overtop the banks, it’s difficult to predict where the flood water will go.
Generally, the District only removes trees that have already fallen in the river or are about to fall in the channel. If a tree is ready to fall, the District often cuts the trunk 2 to 3 feet above the ground and leaves the root in place to help keep the bank stable. Woody debris needs a drying period prior to burning. The District places wet woody debris outside of the river channel, often in piles designed to provide temporary wildlife habitat.
The District clears different areas each winter with the goal of clearing the entire length of the river within District boundaries every 3 years. However, the pace of work depends on river conditions each year - we only work in the river when flows at Glenwood Bridge are below 350 cubic feet per second. While we're out there working we also remove and dispose of tires, trash and other debris.
Channel maintenance is completed under permits from the Idaho Department of Water Resources and the Army Corps of Engineers, and consistent with a protocol for tree and brush removal that is approved by the agencies. The Idaho Department of Environmental Quality approves debris burning each week based on weather conditions and stops burning any time air quality is potentially at risk.
Rivers naturally move laterally over time. Sometimes this movement can put property at risk when banks destabilize and erode. The District works with property owners to stabilize eroding banks by placing rock in the river and along the banks to redirect flows and reduce erosion. Generally, this work is requested and largely funded by the property owner, but guided by the District to ensure an effective outcome. The District plants willows in or immediately behind rocks placed along the bank to further stabilize the banks and reestablish vegetation.
From time to time, the District works with highway districts to remove gravel at key locations. Accumulated gravel can alter river flow and present a significant risk during a flood. Accumulated gravel is removed from the channel and used by the highway districts for construction projects, which saves taxpayer money.