Not the best to wash in water others are going to drink/use lower down. Ultra PC I know but you will be washing off fecal matter at least. Take the water and sluice off away from the water source;200 ft (70metres away)
It does depend on what you are going use it for and maybe for how long.
I used successfully an Equinox poncho shelter. I now have a Cuben fibre ZPacks poncho as rain wear. Both are not easy to put on. With a poncho tarp you need to be quick setting up in the rain as you are taking your rain gear off to put it up as the shelter.
In trees I found it easier and quicker to drape the poncho over a ridge line for the best set up; but there are other configurations
If you are going lightweight that sort of gear suits and you soon will gain experience if the weather is not good.