I don’t really think I’ve ever heard someone refer to a “mouth” of Lake Pepin.
The “Head of the Lake” that most refer to when talking about postspawn fishing is generally the area up river from Frontenac as the river takes a East-West turn to the area where the defined channel begins. When people refer to the “Foot of the Lake” they’re generally talking about the area below Camp Lacupolis and up river from Reads Landing.
As far as navigating the river now, I’d stay away all together. Even the local guides are taking a break until the water crests now as it’s hard to find a launch that isn’t flooded and the debris in the water is becoming more widespread. A friend sent a picture of a full picnic table going down the middle of the channel on Monday.