Well, yesterday evening I FINALLY got the culvert free of ice. The pond water was just so cold!!! that it took a long time.
Using just the running water from that sump pump thawed all but the last maybe 3 feet. But the icy pond water was too cold to thaw the pond/north end of the culvert.
So... I connected the hose end to the HOUSE faucet, turned on the hot water (we have an on demand water heater, so endless hot water as long as the well keeps up). But... even with hot water, and the hose stuffed up as far as it would go in the culvert (on the south end, so as not to add water to the pond) the last couple of feet of ice would not thaw.
I then put the hose into the north end of the culvert, and WOW is that pond water icy!! And 20 minutes later it thawed all of the way through! Very nice, and DONE!