The Outer Hebrides

Freshwater Landscape and Geology

The chain of islands that form the Outer Hebrides, the most westerly part of the United Kingdom, constitutes only 1.3% of the UK land mass but contains 15% of its freshwater surface area. Within the islands there are approximately 4000 freshwater lochs and 80 catchments which contain salmon, sea trout, brown trout, charr, eels, lamprey and pearl mussels and of these there are 24 rivers and loch systems with significant importance.

There are 3 Special Areas of Conservation (SAC) incorporated around the Outer Hebrides including one designated for Atlantic salmon in Loch Langabhat.

The islands’ largest loch is Loch Langabhat in central Lewis and Harris which covers an area of 9Km2. Loch Suaineabhal in West Lewis is the deepest loch on the island at 66m and the largest by volume. Loch Bee in South Uist is brackish and tidal, it is a (7Km2) shallow loch of less than 1m in depth. The lochs are often connected by streams and small rivers and most systems are low altitude. Over a thousand streams enter the sea around the islands.Achmore moorland - central Lewis

Much of Lewis and North Uist is characterised by relatively flat open moorland. Mountains of up to 800m high occur to the south of Lewis (Uig and Pairc), North Harris, and the east coast of the Uists.

The coast of the islands encloses extensive sea lochs. These are dominated by narrow fjordic lochs the most prominent of which are Seaforth and Reasort in Lewis and Harris respectively. The Uists contain a number of wide and shallow sea lochs the largest being Loch Maddy in North Uist.

The bedrock of the islands is predominantly Lewisian gneiss. This metamorphic rock is slow weathering and commonly overlain with peat. Consequently our freshwaters are typically acidic and nutrient-poor. However, along the west coasts of the islands, particularly in the Uists and Benbecula, but also including parts of Uig in Lewis, sandy coastal plains have been formed by the mixing of the calcium rich blown sand with acidic peat. In these areas calcareous shell sand dissolves to produce alkaline freshwaters. These fertile waters support more plants and larger resident fish than other areas.