The Afon Ystwyth is 39km long from its source in the upland bogs of the Elenydd east of Cwmystwyth with a catchment area of 186sqkm. The river then flows generally westwards before draining into Cardigan Bay at Aberystwyth.
The Ystwyth valley was once relatively densely populated due to its mineral wealth. Silver, lead and zinc have been mined in the valley since Roman times, an activity that reached its peak in the 18th century. The largest of the very many mines was Cwm Ystwyth Mine, see our Earth Cache GC3J24X.
The River still carries elevated levels of lead, zinc and silver in its water, mostly due to seepage from the abandoned mine workings.
The upland section of the Ystwyth is on very remote mountain wilderness. The first settlement of any size is Cwmystwyth at 8km from the source, followed by Pontrhydygroes at 14km.
Llanilar is reached after 28km. Here the section of the river to the east of Llanilar has been straightened when the Carmarthen to Aberystwyth railway line was built in 1867 (closed since 1964). The railway alignment is now used as part of the National Cycle Route network with a good series of caches along the Ystwyth section.
Llanfarian at 34km and Rhydyfelin at 36km are on the southern outskirts of Aberystwyth where the river is approaching its tidal limit at about 37km. West of Rhydyfelin, the river emerges into what once was a broad tidal estuary, although the land has been reclaimed during the 18th century when the river mouth was diverted to meet the Afon Rheidol at the seaward end of Aberystwyth harbour. This was achieved by placing large boulders now making up Tan-y-Bwlch beach across the estuary and digging a trench through a rocky bar at the foot of Dinas hill.
The rock formations along the entire length of the Ystwyth, along with most of the Cambrian Mountains are Silurian, despite the name of the hill range. The rivers in the area have swapped beds several times due to glacial activity.
The upper Rheidol once was the origin of the Teifi but was tapped into by the Ystwyth due to glacial erosion at the then head of the Ystwyth valley near Pontrhydygroes. Later, the same process happened to the Ystwyth when the Rheidol captured the headwaters of the Ystwyth due to erosion at the Rheidol's head at the time, near Pontarfynach. Both captures resulted in a shorter distance from the source of the Teifi-Ystwyth-Rheidol on Pumlumon to the sea, causing the formation of the Ystwyth and later, Rheidol gorges due to faster-flowing water.
The original valleys of the Ystwyth and Rheidol, i.e. today's lowland sections, run along geological fault lines and the capture points are the points where the fault lines intersect with the original Teifi valley, which can still be seen as a high-level valley running north to south from Pontarfynach (Rheidol) via Pontrhydygroes (Ystwyth) to Pontrhydfendigaid (Teifi).