Of glacial origin, Loch Morar is the deepest lake in the British isles with a maximum depth of 310.0 m and ranks seventeenth deepest in the world. It is elongate in shape with its main axis running in an east-west direction. It has a total length of 18.8 km, a mean depth of 86.6 m and a volume of 2.3073 km³. Though there are two basins both deeper than 270 m, Loch Morar is essentially a single large basin, with, in most places, very steeply shelving sides.

Like Loch Shiel, Loch Morar has moderate watershed altitude, but very steep slope, very little arable ground and base-poor geology. But in this lake, there is much less forest around the lake than Loch Shiel.

The mild winters and cool summers experienced close to the lake are caused by the damping of seasonal fluctuations in air temperature not only by the proximity of the Atlantic Ocean but also by the heat absorption and release of the large volume of water in the lake itself. Significantly ice is limited to shallow areas, even in severe winters.

Loch Morar, with the smallest catchment among the other four Scotland lakes (Loch Ness, Loch Shiel, Loch Awe and Loch Lomond) drains relatively small area (less than one tenth of that of Loch Ness). The lake, therefore, is likely to be very nutrient poor (1, 2, 4, 10).