Limestone sands
If a small stream has a low pH but low levels of dissolved metals, alkaline limestone sand can be added to the stream's banks. The sand will gradually wash downstream: most of it dissolves in the water, but some undissolved limestone may become incorporated into the stream bed. Periodic flooding helps keep limestone sand moving downstream, but also makes it necessary to add more as the sand is exhausted.
Limestone Sand: Pros and Cons by Penn State School of Natural Resources and Center for Watershed Stewardship