Bill - Fishman was on target; one of the aquatic glyphosate formulations (ie. Rodeo, Aquamaster, AquaPro, AquaNeat, etc. etc.) will do the trick with a mid to late summer treatment. 2,4-D also works well on willows with early & mid-season treatments, but requires a license in TX and has greater risks of damage to non-targeted trees and adjacent crops - if any.
Also, a new aquatic label is expected in the near future (@2nd qtr, 2003?) for an awesome compound to control willows, cattails, certain lily spp, hyacinth and several other plants that may present a problem in aquatic sites. No product names mentioned yet - to protect the rambunctious from themself. Will post more details when full registration is granted by EPA.