Buffering of the salinity intrusion in estuaries by channel convergence

Abstract. A one-dimensional advective-diffusive model is used to investigate the influence of channel convergence on the runoff-dependence of the distance salt intrudes from the ocean into estuaries. We express the runoff dependence of the dispersion coefficient as K~ρ^β, and that of the intrusion extent as x_s~ργ, where ρ is the normalized fresh-water discharge into the estuary, and show that β+γ=1 for a prismatic channel. For a channel that is narrower at the river end we find that for relatively low runoff, β+γ<1. Using two decades of salinity observations in the Chesapeake Bay, and Delaware Bay and a shorter data-set for the Connecticut River, we show that channel convergence may contribute significantly to buffering the salinity intrusion. We demonstrate that in a well-mixed estuary with significant convergence, the geometry alone can explain the relatively weak response of the salt intrusion to fluctuations in river discharge. In contrast, a less tapered, but more stratified estuary dominated by gravitational circulation will respond more strongly to runoff fluctuations.