JMaul is correct. Sunday River.
I believe Sel was also a stockholder
ceo actually listed the rivers correctly -- in order (!) but neglected to offer Sunday River as a guess. I almost gave it to him for that.
NES13 correctly points out that the Androscoggin is one of five rivers that flow into Merrymeeting Bay, however that is considered part of the Kennebec -- which I personally find to be very insulting to the Andro -- and it is the Kennebec that continues and flows into the Atlantic. Wikipedia gives an overview of this if you really care, and the USGS map of the Kennebec drainage system shows it.
the upstream World Cup was first held at New Hampshire's Nansen Ski Jump in 1939 or 1940 (sources don't agree) and the downstream was the Nordie World Cup held at Black Mountain of Maine in 1950. Both drain into the Androscoggin. The snowmelt from the old Sugarloaf world cup Alpine events went into the Kennebec as ceo pointed out.
I believe Sunday River has also hosted some sort of WC competition, moguls or something.
The lost lake is explained on signage at the top of Locke Mountain:
Glacial Lake Bethel.
the two New England ski areas with the river relation are Sunday River and Mad River Glen, which I believe are the only two with "river" in their names. I could be wrong about that.
Thanks all who participated or looked in on this madness!
JMaul, take it away....please