Spatial and temporal variability of seabed disturbance in the York River subestuary

Seabed and water column observations from the microtidal York River subestuary were used to examine temporal changes in this fine-grained environment and compare the impact of infrequent disturbance events (annual to interannual time scales) with more frequent events (fortnightly to seasonal time scales). An experimental across-channel transect is described by time-series observations of seabed radiochemical profiles, X-radiographs, sidescan sonar, and current meter data for 1999. Four morphological subenvironments were distinguished based on the average maximum 137Cs penetration depth and the character of excess 210Pb profiles: southern shoal, flank, secondary channel, and main channel. Various combinations of long- and short-term events (e.g. spring–neap tidal cycle, spring freshet, storms) influence subenvironment sediment structure and physical mixing depths. Shoal sediments were eroded/deposited on the spring–neap tidal cycle during the early spring months, whereas the other three subenvironments were not noticeably influenced by tides on a fortnightly basis. Shortly after the peak currents of the annual spring freshet, sediments were temporarily deposited on the shoal and flank. Major events, such as Hurricane Floyd, did not affect this section of the river except as a result of subsequent longitudinal sediment transport along the estuary main channel that resulted in rapid sediment erosion and deposition of 100 cm in 1 month. The formation and infilling of longitudinal furrows have previously been implicated as contributing to shallow (25 cm) seabed mixing in the secondary channel. However, interannual variations in river discharge and estuarine circulation likely control sediment supply to this section of the river and may augment the location and frequency of furrow formation within the secondary channel and the main channel. Overall, the data indicate that the seabed at the upper York transect is frequently physically disturbed such that bioturbation structures are not preserved in surface sediments (0– 30 cm). However, the presence of bioturbation structures below 30 cm in some of these subenvironments suggests either that this section of the river has experienced long-term changes in the dominant mixing mechanism or that the sedimentary stratigraphy is event-driven.