Lagrangian Statistics of GPS Surface Trajectories in Two Natural Rivers

GPS-equipped surface drifters were released in three configurations on four hydraulically unique reaches in two rivers: Skagit River, WA, USA and Kootenai, ID, USA. GPS drifter measurements are described and Lagrangian statistics are applied to investigate and separate natural river transport from mixing processes. Drifter releases were conducted on two relatively straight reaches and two meandering reaches. Straight reaches resulted in spreading proceeding faster than quadratically in time. This observation suggests a intermediate field mixng regime where both velocity shear and random processes are important, no constant value of dispersion was observed. The meandering reaches resulted in spreading which increased quadratically in time initially, followed by spreading controlled by river shape. GPS driter pair observations from a meandering reach with nearly uniform curvature and spacing indicate a pair separation distance dependence on spreading, consistent with river shape controls. A meandering reach with irregular curvature and spacing did not exhibit spreading dependence.