At first glance I had thought female Mallard or Black Duck; but then I noticed the very dark bill which neither have. Also the white at the base of the bill was suggesting 'teal' to me. David Sibley's, Guide to Birds (p.87), points out the "dark eye-line and white eye-arcs" of the female adult Blue-winged Teal, both of which this duck exhibts. We could hear other duck sounds, closeby in the thick marsh grasses alongside the open waterway. This appears then, to be a female Blue-winged Teal with young nearby.