Saltwater River Freeze and Thaw
I suspect that rather than an ice jam as suggested in the comments, that this was a consequence of sustained high winds that happened to be blowing in just the right direction, thereby blowing the waters away from the side of the bay to which the inlet / bayou / saltwater river is connected.

Negative storm surge can drain shallow bays, sometimes dropping sea level by several feet. This happened, for example, along the west coast of Florida prior to Hurricane Irma. (That negative storm surge became a positive storm surge when the winds shifted direction.) This happens with great regularity where I live. The bayous later today and tomorrow will be very, very low due to a cold front that is pushing through right now.