Maybe I am lucky???? I have been using glyphosate for the last two years to control weeds around and in the pond. I have had no adverse impacts to any type of fish/frogs/ crayfish..... The most beneficial use is the control of emergent weeds when they reach the surface. I mist the covered surface area and two weeks later they have died and fallen to the bottom. I try to divide the area treated into sections to mitigate the impact on decaying matter on the oxygen levels. Glyphosate is a nerve agent and can be harmful in large doses; the chemist I have consulted with feels the diluted amount out of a sprayer in conjunction with the huge dilution once it hits the surface would be unlikely to harm fish. In treating the weeds around the pond you spray when the weeds are tall and the frogs and other small animals never come in contact. Once the glyphosate reacts with the plant material it loses its harmful effect so even my grass carp have no impact when they eat the leftover material. I would spray the willow and watch it die off and then cut it down