Anglers can spot small, dark stone flies against the white background of snow, covered stream banks in January and February. Cottonwood leafs pack the bottom of the Bitterroot and Clark fork rivers and are a prime food source for these bugs. Winter stonefly nymph’s are responsible for the major portion of the breakdown of leaves and small particles Winter emerging stoneflies typically leave the water on sunny days during the afternoon when the most heat is available. Most Winterstone flies are poor fliers. They would rather run than fly some have severely truncated wings that prevent them from flying at all. Skwala nymph stoneflies are about 3/4 inch long and prefer fast and cold water.
Birds swooping above the water is a sure sign the Blue Winged Olive or March Brown mayfly is hatching. The Bwo dlunn’s emerge in early spring on humid and even rainy days. Mature nymphs that are ready to hatch, rise through the column just underneath the surface of the water propelled by gases that build up underneath their exoskeleton. The March brown hatch typically follows the Bwo hatch. In my experience the March Brown dunn's hatch on the Bitterroot River just after lunch. Clouds of spinners can be seen later in the afternoon following the hatch of the dunns. Fishing a size 12 or 14 parachute in brown color is very productive and easy for anglers to see on the water.