This week we had a very scary incident on the St. Marys River. Thanks to boatnerd.com for the updates. I don't know who is writing those updates but I really appreciated the information.
The Edwin H. Gott, a 1,000 footer, went aground just south of Neebish Island. It took 4 tugs to free her but it looks to me like the water level in the St. Marys River was a help too.
The water level of the river can raise and lower frequently during the course. From living on the river, I have seen a correlation between the wind direction and sometimes the atmospheric pressure and the water level.
The few inches that the depth fluctuates is not an issue in most of the river but in the Rock Cut it is definitely important. It is not unusual for a ship to be waiting up from the Rock Cut because the water is too low. You can hear the ship's captain on the scanner asking for the water level in the Rock Cut.
You can follow the level in the Rock Cut on-line. The National Weather Service provides a Water Level Monitoring Station. There is a list of locations on the left. Scroll down to Rock Cut, MI and clip on the number to the left of it. The first chart is water level.
To read the chart, observe the red line. When it is above 0, the water is above datum. Datum is where the level needs to be for safe shipping. I'm sure an engineer could give a better explanation for datum, but that's the bottom line.
There is other information available there too such as wind direction and air pressure.
Early Wednesday morning the water level dipped 11 inches. That may have had something to do with the grounding of the Gott. The Gott wasn't freed until the water level raised.
Very interesting for you river watchers.