The river has its source in Lough Derg, in County Donegal, Republic of Ireland, and it flows into County Tyrone, Northern Ireland, east through Castlederg to join with the River Strule to form the River Mourne.

[1] The River Derg's length is 28 miles (45 km).

[2] The River Derg is known for its salmon, grilse and sea trout.

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