Because the sand is wet for long periods of time would that kill most of the weed seeds in it?
some seeds maybe, but there are seeds that have adapted to just about every condition nature can throw out.
don't worry about weeds as much, it turns most of them are actually useful plants.
if you would like to try the sand, which could be fine or not fine depending on what is upstream( some people dump horrible things into waterways) simply mix a gallon or so of soil containing the sand and put a cheap extra plant in there. take note of any ill or beneficial effects, its as simple as that.
For all things come from earth, and all things end by becoming earth.