The source of the 75 mile long Towy 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).

The Towy has long been recognised as supporting the best seatrout fishery in Britain, and possibly Western Europe. Although runs have declined a little in recent years, the 10 year average declared rod catch is still 5000 fish. Salmon catches over the same period average some 800 fish, placing the Towy high in the listing of Welsh salmon rivers.

It is renowned for its big, fast growing seatrout (also known as sewin). Multi-spawners, they can reach well over 15 lbs in weight. The largest fish enter the river in March and April, with sizes gradually declining through the main runs of 3 - 6 pounders of May and June to the shoal fish of July and August. An instream cage trapping survey carried out by the Welsh Water Authority during the late Eighties suggested that some 20,000 seatrout of average weight exceeding 6 lbs had entered the river by the end of May.

Although fresh seatrout continue to enter the river during September and October, many of the earlier run fish, by mid September, are becoming gravid and preparing for spawning. By mid to late October, they will have travelled to the furthest, tiniest, tributaries. Each hen fish can be attended by several males, ranging in size from small precocious trout to large cock seatrout. The same group of males will await, and accompany a succession of hens to their spawning beds.

While salmon run throughout the fishing season, the main runs occur mostly during August, September and October, but are largely dependant on flows. Summer spates can produce good runs of grilse. Following seven years of liming in Llyn Brianne, the upper Towy is thought to be recovering sufficiently for a return to successful spawning in the 10 mile stretch above Llandovery. This should bolster the earlier runs of salmon and large seatrout.

There are some reasonably sized brown trout in the main river. These are generally regarded by local anglers as a bonus by-catch. Small trout are present throughout the catchment, but being indistinguishable from juvenile seatrout, they present a problem to responsible fishermen. The minimum size limit for trout (and seatrout) has recently been raised to 9 inches.