Of glacial origin, Loch Shiel is the fourth largest lake by length in Scotland and this is one whose water regime is among the least affected by man. The lake has a total length of 28.0 km - only exceeded by Lochs Awe, Ness and Lomond. Like Loch Awe, it is very narrow with a mean breadth which is only 2.5% of its total length. The main upper portion of the lake runs in a northeast/southwest direction, but about 8 km from the outflow the axis bends and the lower portion bends almost due west. The surface area of the lake is 19.6 km²: it has a mean depth of 40.5 m and a maximum one of 128.0 m. It contains a total volume of 0.8925 km³.

Like Loch Lomond, the northern end of Loch Shiel is much deeper than the southern end. The basin is essentially a single one, but there are a number of minor depressions here and there, two of them deeper than 120 m.

Since the construction of the River Leven Barrage in Loch Lomond, only Loch Shiel has a natural outlet. Loch Awe is regulated by a barrage: Loch Ness is affected by the works associated with the Caledonian Canal and the short outlet channel from Loch Morar is influenced by a small hydro-electric scheme.

Loch Shiel has very similar catchments to Loch Morar in many respects; moderate altitude, but steep slopes, very little arable ground and base-poor geology. The lake, therefore, is likely to be very nutrient poor (1, 2, 4, 10).