The River Deel near Crossmolina is one of Ireland's most diverse and rewarding salmon and trout rivers, rising high in the Nephin Beg Mountains of County Mayo and flowing approximately 45 kilometres through wild moorland, open pastures, and the town of Crossmolina before entering the northern end of Lough Conn. As the largest tributary of the River Moy system, the Deel offers an exceptional range of angling experiences — from delicate dry fly fishing for brown trout and classic spring salmon fishing in the lower reaches, to spirited brown trout and grilse action in the remote upper stretches. The river drains a catchment of approximately 63 km² and forms an integral part of the River Moy Special Area of Conservation (SAC, site code 002298). In the vicinity of Crossmolina, the river passes through an area of karstic limestone geology, where it seasonally sinks underground for a reach of approximately 4 kilometres between Ballycarroon Ford and a point downstream of Crossmolina — a unique geological feature that shapes the river's character and fishing dynamics considerably.