with a few exceptions, tree roots in the landscape rarley go deeper than 24 inches. In the forest they may go much deeper but not in your yard, unless it is an old farm field. In any case, willow roots are never, ever going to reach a depth of 6-7 feet.
Toad
I think the only exception to that rule is if it follows a pipe or drain system down to where there is more water. For example, a catch basin with an underground pipe running to the storm drain/dry well. The roots don't have to penetrate the pipe, they'll happily follow the outside.
-al sung Rapid Realm Technology, Inc. Hopkinton, MA (Zone 6a)
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