Determination of bedload transport using naturally magnetic tracers: First experiences at Squaw Creek, Gallatin County, Montana

Abstract

Pebbles and cobbles constitute an important part of the sediment discharge of many streams. Data on the movement of this coarse bedload is important to a variety of studies including analysis of total sediment yield, resevoir‐filling rates, bridge erosion, and trout‐spawning habitat. Despite the importance of knowledge regarding coarse bedload transport, measurement remains difficult and unreliable. A new detector has been constructed which records the movement of naturally magnetic pebbles and cobbles. The detector was installed at a log sill in the bed of Squaw Creek, Gallatin County, Montana, to evaluate the detector under natural conditions. During a flood in May 1981 the detector recorded the passage of magnetic particles larger than 32 mm. Extrapolation of the data allows estimation of total bedload transport. Based on the composition of an unvegetated gravel bar upstream of the detector and on measurements of turbidity during the flood, bedload accounted for approximately 66% of the solid material leaving the drainage basin. Since many coarse bed streams probably contain material as magnetic as the andesitic rocks in Squaw Creek, the procedure should be widely applicable.