It extends about 200 miles inland from south to north. The bay is from 4 to 30 miles wide and as much as 348 feet deep. Average depth is 27.5 feet. The James, York, Rappahannock, Potomac, and Susquehanna are the biggest rivers emptying into the bay which has shoreline of some 3,600 miles. The mixing in the estuary of sea water and fresh water creates salinity variations.
Salinity is a function of freshwater discharge and tide velocities.
Salinity patterns vary seasonally with the amount of fresh water inflow. Due to seasonal variation and the density difference between fresh and saline waters, significant non-tidal circulation often occurs within the bay. Two high and two low tides occur every day. The resident species of plants and animals are able to adjust to the changes in salinity unless droughts or floods persist for a long time. The effects of temperature are important because the bay is relatively shallow.