Based on my experience fishing the Skwala hatch on the Yakima River for many years, from mid-February through mid-April, you will not see a lot of them on the water or flying. Stonefly nymphs crawl ashore to molt into the adult form where they seek shelter and mate in the grass and brush along the shoreline. They become available to the fish when the females fly out to land on the water to oviposit or when males or females fall into the water (the Skwala's primary defense mechanism when disturbed is to let go and drop; hopefully deeper into the brush or grass, but often into the water). This occurs so often that trout will seemingly line up along the shoreline waiting for it to happen (hint: fish tight along those long banks, even if the water seems a bit shallow).