SEASONAL MEASUREMENTS OF SEDIMENT ELEVATION IN 3 MID-ATLANTIC ESTUARIES

Relative sediment elevations have been measured seasonally since Sprin g, 1990, in three estuaries representing a range of estuarine geomorph ologies, sizes, and watershed inputs: the North Inlet estuary, South C arolina (SC), the Patuxent River estuary, Maryland (MD), and the Rhode River estuary, Maryland (MD). Sediment elevations were quantified usi ng a levelling-arm device that allowed accurate, repeatable measuremen ts at four arm orientations per site (with nine replicates per orienta tion), a number of habitat sites per location, and a number of locatio ns in each estuary. This hierarchical design allowed variability to be partitioned into nested spatial scales. At North Inlet, we establishe d six locations characterizing a range of freshwater and oceanic influ ences, and marsh ages. Water level gauges continuously recorded inunda tion rates at three of the six locations. Six sediment elevation locat ions along the Patuxent River estuary (one habitat site each) differen tiated tributary marshes from main channel marshes, marshes from mudfl ats, and upper from lower river geomorphologies. Seven sediment elevat ion sites in the Rhode River estuary were all within the same 0.25 ha area (spatially equivalent to a single habitat site in the other two e stuaries), and we used these data to investigate small-scale variabili ty in brackish marsh sediment elevations over time. Results from North Inlet showed sediment elevations increasing at the greatest rates at locations nearest to freshwater influence. These rates were two to thr ee times apparent sea level rise (ASLR), which is 2-4 mm yr-1 in all t hree estuaries. Sediment elevations in dead end tidal creek marshes-th ose without direct freshwater inputs-generally increased at rates comp arable to ASLR. Geologically older marshes showed little or no net acc retion over 2.5 years. A large decline in sediment elevations (-9.4 mm yr-1) at a mudflat located in the headwaters of a geologically young tidal creek appeared to represent the ''birth'' of a transgressively s ubtidal creek from an intertidal mudflat. Sediment elevations generall y increased at Patuxent River tributary marsh sites, but at greater ra tes in lower river than upper river tributaries. In contrast, a lower river main channel marsh and upper river mudflat appeared to be erodin g while a middle river marsh and mudflat appeared to be accreting. Thi s pattern suggests that the extensive mid-channel tidal freshwater mar shes of the upper river and the isolated tributary headwater marshes o f the lower river may be sediment sinks, while others may be sediment sources. The Rhode River study, on the other hand, showed that small-s cale variability in brackish marsh sediment elevations may be much gre ater than seasonal or long-term differences. The consistent use of thi s accurate and repeatable technique to quantify estuarine sediment ele vation dynamics in a number of systems will continue to generate data critical to future comparisons of Atlantic and Gulf Coast estuaries.