The source of the River Towy rises at an altitude of 425m AOD in the Cambrian Mountains, falling steeply for the first 17 km before reaching Llyn Brianne reservoir. Down stream of the reservoir the gradient of the Towy lessens just above Llandovery, from here the river begins to meander across its flood plain, before reaching the tidal limit just upstream of Carmarthen.

Many tributaries join the Towy on its course to the sea. On the left bank, the Bran joins near Llandovery. Joining the river at Llangadog, the Sawdde flows off the Black Mountain, the wettest area in the catchment. The Cothi is the largest tributary of the Towy, and joins the main river close to the tidal limit. Another major tributary, the Gwili, joins within the tidal reaches.

From its source to the sea, the Towy is 111 km in length and drains an area of 1,376 km2. Average annual rainfall for the catchment ranges from 1200mm on the coast to 2400 mm in the Black Mountain. The average daily flow at Capel Dewi is 38 3 m ;ð/ s. The flow of the main River Towy downstream from Llyn Brianne is regulated, with releases from the reservoir timed to coincide with the major abstraction at Nantgaredig, providing water resources to much of South Wales.

The catchment is predominantly rural with intensive dairy farming concentrated in the lowlands. Mixed dairy and livestock rearing are the major uses around the middle reaches, while in the upper catchment, land use changes to sheep rearing and coniferous forestry.

Much of the urban development has centred along the flood plain of the middle and lower reaches, with the main towns of Carmarthen, Llandeilo and Llandovery being located adjacent to the Towy itself. The population of the catchment is estimated at

41,900 (1991 census). The A40 runs along the river from Llandovery to Carmarthen.

The catchment is located on Lower Paleozoic strata ranging in age from Ordovician through to Carboniferous strata. Rock types represented are for the most part Ordovician mudstones and shales, with outcrops of Devonian Sandstones and Carboniferous Limestones. Drift deposits occur within the main river valley.

Parts of the upland catchment suffer from low pH and associated elevated aluminium and zinc concentrations due to surface water acidification. This surface water acidification is brought about by a combination of air pollution, afforestation, and base poor geology in the area. Tributaries on the left bank of the Towy, such as the Sawdde, do not suffer from the effects of acidification, having pHs of around seven. Direct liming of Llyn Brianne began in 1991, as a pilot study to assess the feasibility of raising the pH of the water and alleviate the effects of acidification. Since 1996, liming has been carried out by two purpose built lime dosers, which apply frequent measured doses of lime to the two largest streams that flow into the reservoir.

Water quality in the main freshwater river is generally excellent with 100% of classified reaches being in chemical Class A or B in the 1995 General Quality Assessment survey (see Appendices). Biological quality was also high, but poorer in the uplands due to acidification. The biological quality of the Towy has increased markedly below Llyn Brianne, from biological classes D to A since the liming operations began. High quality water in the Towy must be maintained to protect important uses such as the potable water supplies and salmonid fisheries.

The River Towy supports many nationally rare and protected species. Recently (February 1998) the Main River Towy from Llandovery to the confluence with the River Taf and Pembrey coast Site of Special Scientific Interest (SSSI) Carmarthen Bay was designated a SSSI for its assemblage of flora and fauna. Significant species include the allis and twaite shad, otters, little ringed plover, freshwater pearl mussel, and other invertebrates,. The River Towy has also been put forward as a candidate Special Area of Conservation (SAC) under the European Union (EU) Habitat and Species Directive because of the presence of the two shad species. The proposed SAC is down stream of the A40 Road Bridge at Llandovery to the mean low water level.