Abstract
We describe the flow paths of negatively buoyant river inflows in a small lake with multiple basins separated by sills (Toolik Lake, Alaska) using field data and three‐dimensional simulations. Comparisons of field observations, analytical computations, and simulations show that in small basins in which the timescale for filling (τ f ) is less than the timescale of an event (τ 0 ), overflow is the dominant mechanism of interbasin exchange. Exchange is mediated by internal wave displacements if upwelling reaches the height of the sills, z s . This criterion is met if the Lake number, the inverse of which is proportional to the degree of metalimnetic tilting, is less than z i /( z i − z s ), where zi is the vertical displacement of the intrusion. Entrainment followed by horizontal dispersion is the dominant mechanism of interbasin transport and can account for ~65% of the exchange between large subbasins. Entrainment is enhanced when submerged sills force river water to flow close to the surface layer. Depths of intrusions depend upon discharge. They occurred in the lower metalimnion for the coldest events analyzed but were near the top of the metalimnion during low‐discharge events, such as during brief cold fronts. Persistence of intrusions depends upon the time interval between cold fronts; they ranged from a few days to three weeks. Timescales of horizontal mixing vary with meteorological forcing; they ranged from on the order of 1 d for wind events with speeds of up to 6–8 m s −1 to on the order of 10 d for light winds.