the USGS gauge is at the Little Falls dam, which is NOT tidal, meaning that the water level only depends on the amount of water coming down the river. More water (the other graph in CFS ,cubic feet per second), higher the gauge level
Little Falls dam is the alternative water intake for the Washington Aqueduct when Great Falls is not in service.
For high and low tides you have to check a place downstream, like the Washington channel, and add 10-20 minutes (? I guess, people help me out here) because the tide will influence a place upstream a bit later. You can also figure that the difference between Alexandria and Washignton channel in minutes, should be pretty much the same as between Washington channel and Chain Bridge.
the force be with you.
M.