Seathwaite Tarn is one of the largest of the tarns, with a surface area of some 66 acres. Like many of the larger tarns it isn't entirely natural. In this case the tarn was enlarged by the construction of a dam in 1904, and is used to provide drinking water. The dam itself is one of the longest in the area, at 36mm long. It had to be repaired extensively in 2010.
As a result of the dam the tarn fills the entire valley flow between Grey Friar to the north and Dow Crag to the south, with footpaths along both shores. Just about the only flat ground left in the valley comes just above the tarn, leading up to the remains of a disused mine. At the head of the tarn the valley narrows right down, with Raven Nest How jutting out almost into the tarn.
The tarn itself is long and narrow, with a largely strait northern shore and more crinkled and steeply sloped south shore. The tarn is the source of Tarn Beck, which flows west from the tarn before turning south just before reaching the Duddon. The two watercourses then run parallel to each other for some distance, before eventually merging just past Seathwaite Hamlet.
How to Reach
Seathwaite Tarn can be reached from the Duddon Valley. There are two main routes in. The first comes from the small Forestry Commission car park at Froth Pot. Head north along the road from the car park for about a third of a mile then turn right onto a signposted right of way (the second you will pass). This runs through woodland at first, then along the side of a stone wall. The path then crosses the wall and turns left to climb the valley of Tarn Beck to the tarn.
The second route comes from Seathwaite Bridge, where the road up the valley crosses Tarn Beck just north of Seathwaite hamlet. Take the minor road that branches off to the right just south of the bridge. This eventually turns into the water company track which cuts across the fellside to reach the tarn.
Tarn Head Beck flows into the eastern tip of the tarn.
Bleaberry Gill flows into the south-western end of the tarn, after rising on the slopes of Dow Crag.
Tarn Beck flows west out of Seathwaite Tarn, then after just under a mile turns south and runs parallel to the River Duddon for some way.