Metamorphic rocks will not greatly affect the water chemistry of your testing site. Neither will igneous rock, most common of which is flint and granite. Also, if rocks are present in your stream creating riffles, more oxygen from the air would be drawn into the creek water, providing an ideal conditions for aquatic life. Rocks also add to the architecture of the riparian zone lining the creek, which promotes land stability and less erosion. If soil erodes into the creek, it can change the water chemistry.
Geology is definitely an important aspect when learning about our waterways.
After rinsing all containers, pour the contents of a bailer or shoreline sampler into Sample A bottles. Then bail again. If you fail to get enough water to ensure completion of all your tests, then bail again. Don t come back 15 minutes later for more water. You take an A and a B sample in order to test two trials to make sure your results are accurate.