South Boulder Creek Colorado watershed mallard migrations

Hiking West Southern Boulder Creek here in Boulder Valley around Feb - April, I’ve often observed overflights of hundreds of ducks (usually mallards, some red heads) and Canada geese just before dusk in groups of 2-20 headed east or downstream above the creek corridor. Any information on the larger migration routes, species, numbers, destinations, origins, stopovers, trends, etc.?

I believe eBird has more info about migration routes, etc.