IRREVERSIBLE WIDENING OF A SMALL ESTUARY: WHY?

Repeated surveys of three sections across a narrow estuary on the coast of Maine document little change in lateral position or depth over a period of nearly a decade. In the reach studied, the estuary is 3 to 5 m wide, about 1.4 m deep, and has near-vertical banks composed of reddish to grayish peat with varying amounts of clastic mud.

Prior to initiation of the surveys, bank collapse had occurred at several places. One of the survey profiles crosses one of these collapsed blocks. The block is about 1.7 m wide and 5 m long, and its surface is about 1 m below the general level of the salt marsh. When first surveyed, there was a crack about 0.3 m deep near the middle of the block, suggesting tension and lateral translation toward the channel. Over the period of the study, the crack has filled but otherwise sedimentation on the surface of the block has been negligible.

If the width of the estuary prior to bank collapse was a steady-state width, widening on one side by bank collapse would have to be compensated by deposition on the other side to maintain the width. As the opposite bank is vertical, such deposition is improbable. Alternatively, sedimentation on the collapsed block could build it back up to the general level of the salt marsh over time. However, the surveys, together with observations of other blocks nearby, suggest that sedimentation rates on these collapsed blocks are orders of magnitude too low to keep pace with the rate of collapse. Thus, the estuary is widening irreversibly. While such widening might be attributed to the presence of a culvert near the mouth of the estuary, this seems unlikely as the culvert is large enough to accommodate the flows quite easily, the area of bank collapse is over 100 m from the culvert, and there is no obvious mechanism for such an effect. Nor is it clear how the accelerated rise in sea level during the 20th century could result in such widening. Thus, the cause of the widening remains enigmatic.