On a hazy winter day this past December two tugs pushed two scows back and forth across the glassy bay between the Redwood City shipping channel and the shallows off Eden Landing. What looked like your ordinary harbor dredging project, with an orange clamshell clawing up mud to make way for deep-drafting ships, was actually quite extraordinary. That’s because the destination of the mud-loaded scows was not a disposal site but a mile-long, 138-acre stretch of shallow water near Whale’s Tail marsh. This particular stretch of shallows has been carefully chosen by scientists and computer models because local conditions here — tides, winds, wave direction, depth, proximity to marshes — should all help deliver the sediment to the marsh and recharge mudflats. And goodness knows they need it. With sea levels rising, many San Francisco Bay marshes will eventually drown.