Remote sensing of suspended sediment concentration and hydrologic connectivity in a complex wetland environment

Abstract

Maintaining the ecological diversity and hydrologic connectivity of freshwater delta systems depends on regular recharge of floodplains with river water, which can be difficult to observe on the ground. Rivers that form deltas often carry large amounts of suspended sediment, but floodplain lakes and wetlands usually have little sediment in suspension. Remote observation of high sediment water in lakes and wetlands therefore often indicates connectivity with the river network. In this study, we use daily 250-m MODIS imagery in band 1 (620–670nm) and band 2 (841–876nm) to monitor suspended sediment transport and, by proxy, hydrologic recharge in the Peace–Athabasca Delta, Canada. To identify an appropriate suspended sediment concentration (SSC)-reflectance model, we compare 31 published empirical equations using a field dataset containing 147 observations of SSC and in situ spectral reflectance. Results suggest potential for spatial transferability of such models, but success is contingent on the equation meeting certain criteria: 1) use of a near infrared band in combination with at least one visible band, 2) development based on SSCs similar to those in the observed region, and 3) a nonlinear form. Using a highly predictive SSC-reflectance model (Spearman's ρ=0.95), we develop a twelve-year time series of SSC in the westernmost end of Lake Athabasca, observe the timing and sources of major sediment flux events, and identify a threshold river discharge of ~1700m3/s above which SSC in Lake Athabasca is clearly associated with flow in the Athabasca River. We also track the influx of Athabasca River water to floodplain lakes, and in three of the lakes identify distinct discharge thresholds (1040m3/s, 1150m3/s, and 1850m3/s) which result in lake recharge. For each of these lakes, we find a statistically significant decline in the threshold exceedence frequency since 1970, suggesting less frequent recharge during the summer.