You can easily make a sechi disk to measure water turbity -- an indication of water quality in terms of its clarity.
a 20cm-diameter, 6mm-thick Plexiglas disk with a hole in the middle (this may be cut from a square sheet of Plexiglas)
metal disk with a hole in the middle (to weigh down Secchi disk)
an eyebolt with nuts and washers to fit it
rope or cord- try to avoid cotton, as it stretches
waterproof black and white paint
Read more background about this activity in David Schindler's bio...