The current down the St. Clair River, across Lake St. Clair and down the Detroit River, driven by the river gauges. Tap the water for its speed.
Nobody publishes a current map of this whole waterway, so this one is computed. The riverbeds come from 16,501 depth soundings on NOAA's nautical charts. The Canadian half of Lake St. Clair, which those charts leave blank, comes from NOAA's lake-floor grid. The water is routed through that bed the way friction would spread it: deep water carries more and runs faster.
Nothing was tuned, so the checks are real ones. The U.S. Geological Survey measures the water's speed at Port Huron and at Detroit. Over the last month it averaged 3.9 feet a second at Port Huron and 2.65 at Detroit. The model's own answer for those two cross-sections is 3.8 and 2.5. The model also needs only an ordinary roughness for a big river to lose the 6.4 feet of height the gauges show between Port Huron and Gibraltar.
The dredged shipping channels show up as the fastest water, and that is real. In July 2002 the USGS and the Army Corps measured the current across the whole Detroit River from boats. Set against 5,500 spots from that survey, the model is within 3% in water deeper than 13 feet, shipping channels included. It runs too fast in the shallows: about 16% over in 6 to 13 feet of water and 40% over in less than 6.
The flow is live. Each river runs at its own USGS gauge reading, and the difference between the two is added or taken away across Lake St. Clair, which is where it really goes: into or out of the lake's own level. Ten NOAA water-level gauges raise and lower the charted depths.
What the map leaves out is wind. The rivers obey the gauges, but Lake St. Clair is 10 feet deep on average and a stiff wind pushes its surface around. The lake here shows only the steady drift from the delta to the Detroit River, about nine days end to end. There are no eddies either. Line speed on screen is exaggerated in slow water so you can see it move.