Logjams are not jammed: measurements of log motions in Big Creek, Idaho

Nakul Deshpande, Benjamin Crosby

Colloquially, a "logjam" indicates a kinematic arrest of movement. Taken literally, it refers to a type of dense accumulation of wood in rivers widely recognized as bestowing numerous biological and physical benefits to the system but also present serious hazards to infrastructure. Despite this, no in-situ field measurements have assessed the degree of arrest in a naturally-formed logjam. Using time-lapse photography, repeat total station surveys and water level loggers, we provide an unprecedented perspective on the evolution of a logjam in central Idaho. Despite the namesake, we find that the logjam is not jammed. The ensemble of logs progressively deforms in response to shear and buoyant lift of flowing water, modulated by the rising limb, peak and falling limb of the snowmelt hydrograph. As water rises and log drag against the bed and banks decreases, they collectively translate downstream, generating a heterogeneous pattern of deformation. As streamflow recedes and the logs reconnect with the bed and banks, the coherent deformation pattern degrades as logs settle opportunistically amongst their neighbors. Field observations of continuous movement at a low rate are qualitatively similar to creep and clogging, behaviors that are common to a wide class of disordered materials. These similarities open the possibility to inform future studies of environmental clogging, wood-laden flows, logjams, hazard mitigation and the design of engineered logjams by bridging these practices with frontier research efforts in soft matter physics and granular rheology.