I agree with your sign idea.

Many years ago my Dad and I canoed down the Raisin River in Lenawee county. We notified the authorities of our plans. They reminded us to portage around the upcoming falls. There is a sign saying, " three people have died trying to shoot these falls." We told them we'd shot many falls. They were unphased and replied, "No problem. We can change the three to a five."

Signs worked for us. We portaged.