Road and other development in Arizona, Texas, Wyoming, Utah,
Michigan, Washington, etc. etc., etc. are responsible for silting up the
ICW? I don't think so.

Dave, this is getting silly. YOU made that statement, no one else.
"General public" doesn't necessarily mean every citizen in the country. If
the need for government expenditure has to be caused by the actions of each
and every citizen, which seems to be what you are expecting, or accrue to
the benefit of every citizen, there wouldn't be many dollars spent.
There is some validity to your argument--in effect to cut the pork. But I
suspect the ICW can be justified in terms of revenue produced--taxes and
safety to a greater extent than a number of other federal projects.