Lough Arrow is a large spring fed limestone lake with a surface area of 1,247ha located in Counties Sligo and Roscommon in the Republic of Ireland. It is a natural lake with moderate nutrient levels and an intermediate level of ecological productivity. Its nutrient levels have remained largely unchanged over the past 40 years, making Lough Arrow quite rare and hydrologically different from other lakes.
Lough Arrow supports a diverse range of aquatic plants, and is home to the highest densities of breeding Great crested glebe, Merganser and Tufted duck in western Ireland. It is also notable for its Brown trout and Eel populations, and Otter has been recorded at the site.