Maximum ca. 20,000-23,000 years ago but significant ice masses persisted in the Wicklow Mountains for another 4,000-7,000 years. Hydrology Major rivers include the River Liffey, Ireland's 8th-longest river, which rises near Tonduff mountain and flows through the centre of Dublin City, reaching the Irish Sea at Dublin Bay. Its biggest tributary by volume, the River Dodder, rises along the northern slope of Kippure in the far north of the county, while the Kings River joins at Blessington Lakes. Multiple other Liffey tributaries flow within the county. The River Slaney, which starts at Lugnaquilla, flows west and then south before reaching St George's Channel at Wexford town. The Avonmore and Avonbeg rivers join to form the Avoca River at the Meeting of the Waters in the Vale of Avoca. The River Derreen in the south of the county defines a section of the border with Carlow. Most of the county's lakes (or loughs) are glacially derived ribbon and corrie lakes found in scenic trough valleys surrounded by mountains, making them popular with tourists. Notable lakes in the county include Glendalough Upper Lake, Lough Tay ('The Guinness Lake'), Lough Ouler ('The Heart-shaped Lake'), Lough Bray, Lough Dan and Lough Nahanagan. Poulaphouca Reservoir is the largest of Wicklow's lakes, covering in the west of the county. It is the largest artificial lake in Ireland and was created following the damming of the River Liffey at Poulaphouca in 1940. The village of Ballinahown was completed submerged by the reservoir and its 70 families were relocated. Ruins of the old village including buildings, fences and farm machinery can be seen during droughts when water levels in the reservoir diminish. The lower Vartry Reservoir, constructed between 1862 and 1868, is the county's second largest lake. Coast Wicklow has a relatively short coastline, at in length. Wicklow's coastline is mostly straight, with few sizable bays or inlets and no offshore islands, giving it a shorter coastline than smaller counties like Louth and Dublin. Wicklow Head is the county's most prominent coastal headland, and is also the most easterly mainland point of the Republic of Ireland. Wicklow's east coast is a popular domestic summer holiday destination, and the county has numerous beaches including Brittas Bay, Clogga Beach, The Cove, Silver Strand Beach, Sallymount Bay Beach, Ennereilly Beach, Newcastle Beach, Arklow North and South Beaches, Greystones North and South Beaches, Bray Beach, Magheramore Beach and Porter's Rock Beach. Forest