Headhunters Fly Shop in Craig Montana has an excellent post today related to recognizing and protecting trout redds.
A redd is a spawning nest cleared in gravel by the female salmonid (salmon, steelhead, trout). The female forms several depressions in the gravel forming egg pockets into which she deposits her eggs – with the size of a redd dependent on the size of the fish making the nest. While they photograph well from above, they can be difficult for a wading angler to see. Caution and care are the watchwords during spawning seaon.
You can read the post here.