The North Fork of the Boise River is a popular trout stream that originates on the west side of the Sawtooth Mountains and flows roughly 50 miles before joining the Middle Fork of the Boise River upstream from Arrowrock Reservoir. The North Fork flows through the granitic soils of Idaho's Batholith, resulting in the river being relatively unproductive with low levels of dissolved solids and nutrients. This results in very low conductivity water; so electrofishing, which is a common fisheries sampling technique, doesn't work well there. As a result, biologist periodically do snorkel surveys in the North Fork to see how fish populations are fairing.
Despite being relatively "sterile," populations of native game fish in the North Fork of the Boise River include both redband rainbow trout and bull trout, as well as mountain whitefish. Since 1988, the North Fork has been surveyed about once every decade at 16 trend sights scattered from the confluence with the Middle Fork of the Boise River up to the Silver Creek tributary above the Graham Guard Station.
Southwest Region fisheries staff counted fish at all 16 sites during summers in 2016 and 2017. Wild rainbow trout densities are about 1.3 fish per 100 meters squared. By comparison, the South Fork of the Boise River below Anderson Ranch Dam has rainbow trout densities of almost five fish per 100 meters squared.
While relatively low compared to the South Fork's "blue-ribbon" waters, wild redband rainbow densities in the North Fork remained similar to previous surveys, and this population remains stable. Similarly, the mountain whitefish population remains stable, and although bull trout are found at lower densities, their numbers have also been consistent during surveys.
While fish populations in the North Fork Boise River are not as high as more popular fisheries, the North Fork has its advantages and is worth the effort.
It remains a less-crowded option for anglers, who can drive, fly or hike into the different sections depending on road access. The upper section is typically hike or fly-in, and the middle section has a roaded section where car-camping and road-side fishing are options. The bottom stretch is a roadless canyon where people can hike or float to access the secluded waters. All offer consistent fish populations, and the levels of solitude and scenery make the effort worthwhile.