Answer: I'm afraid it might be really difficult to move any plant with a trunk more than a half-inch or so in diameter. You might try moving some of the small "whips", or thin shoots. Generally, smaller plants transplant better anyway, so they'd probably catch up to larger transplants anyway. You might want to wait until the weather cools down in the fall, to minimize stress from hot, dry weather.
Your state's Department of Natural Resources or a similar agency may sell willows and other stream-bank stabilizing plants at very reasonable rates. Try contacting them and see what they have to offer, and when.
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