We had the same problem when we converted what was originally cow pasture right up to the old farmhouse into a garden. We got achappy with a bulldower to smooth it out and scoop out a large hole for a pond. It's about 6 metres buy 4 and was about a metre deep. We left it unlined for f-drainage and it soon filled up.
14 years later it's much shallower after terra forming bullrushes and iris psuedocrus invaded but OH wades in once a year and clears those out. We have frogs and toads and damsel flies and all sorts of other critters including visiting herons, ducks and moor hens. Duck weed has arrived on their feet and blanket weed has presumably come from plants such as marginal irises and marsh marigolds I have introduced but they're fairly easy to clear.
By summer, the pond can be very shallow as it evaporates but we've planted tall miscanthus zebrinus around the edges along with the iris, rodgersias and othet tall plants that mask the view of mud. Most of the year though the water is clear and not at all soupy. This year we've had a dry winter after a very dry 2011 and the pond was empty of water from last April to mid October. The marginal plants have survived though and we have frogspawn again this year.
