File:Afon Tywi south of Moel Prysgau, Powys - geograph.org.uk - 1572540.jpg

English: Afon Tywi south of Moel Prysgau, Powys The river was very low following two weeks of rainless weather. The forested hill of Moel Prysgau is in the distance.

Information from The Carmarthenshire Rivers Trust www.carmarthenshire.org:

The source of the 75 mile long Afon Tywi lies in the Cambrian Mountains of Mid Wales, within 15 miles of the sources of the Severn and Wye, and the smaller Cardigan Bay rivers Teifi, Rheidol and Ystwyth. The upper catchment comprises of afforested undulating moorland. The Tywi Fach rises at an elevation of some 1400 feet above sea level, while the Tywi Fechan rises nearby at 1600 feet.

Just 6 miles downstream, the river flows into the 4 mile long Eastern arm of Llyn Brianne reservoir, while the Afon Camddwr flows into the 3 mile Western arm. A continual release of water from the flow-regulating reservoir marks the start of the main river. The Towy falls quite steeply for 10 miles before flowing out onto the flood plain near Llandovery.

The river then meanders its way along and across the 1 mile wide, virtually flat plain for 30 miles down to Carmarthen and the 12 mile long estuary. With numerous tributaries, the average river flow increases from 157 million gallons per day (mgd) at Llandovery, to 400 mgd at Llandeilo and 855 mgd at Carmarthen (total catchment 515 sq. miles).

The main tributary is the 33 mile long Cothi. With a catchment of 116 sq. miles, the Cothi has an average flow of 120 mgd at its confluence with the Towy. Other significant tributaries include the 7 mile Doethie (20 sq. miles, 35 mgd); 13 mile Bran (50 sq. miles, 71 mgd); 10 mile Sawdde (32 sq. miles, 70 mgd), and 16 mile Gwili (57 sq. miles, 95 mgd).