Rising in the Monadhliath Mountains to the west of Laggan in Highland Council Area, the River Spey flows 100 miles (160 km) east and northeast into Moray where it joins the sea at Spey Bay. It has a catchment area of 3367 sq. km (1300 sq. miles) and on its route to the Moray Firth it passes the towns of Newtonmore, Kingussie, Aviemore, Grantown-on-Spey, Charlestown of Aberlour, Craigellachie, Rothes and Fochabers.
The Spey is the second longest river in Scotland, after the Tay and its headwaters, and the seventh longest in the UK. It is however the longest in Scotland with a single name and, in terms of volume of water discharged into the sea, it ranks eighth in Britain. Much of the river was designated as a Site of Special Scientific Interest (SSSI) in 1998.
Both the upper and lower reaches of the river are fast flowing with steep gradients. In its middle reaches, the river passes through the wide alluvial plain of Strathspey. Here the gradient flattens out and the river has a slow and meandering form. The mouth of the river has always been subject to change with the formation of shingle spits and the cutting of new outlets. Relatively free from pollution and obstruction, the River Spey supports a major spawning population of the Atlantic salmon (Salmo salar) and is one of the most important salmon fishing rivers in the north of Scotland. The sea lamprey (Petromyzon marinus) spawns throughout the lower and middle reaches of the river and the more inaccessible deep water areas sustain one of the largest populations of fresh water pearl mussel in Scotland. The principal tributaries of the Spey in its upper reaches are the Truim, from Loch Ericht, and the Calder. After widening into Loch Insh, the Spey is joined by the Feshie and the Nethy which flow down from the Cairngorms and the Dulnain which flows down from the Monadhliath Mountains west of Carrbridge. Below Grantown-on-Spey the river is joined by right bank tributaries that include the Avon, Livet, Aberlour, Rinnes and Fiddich. During the 18th and 19th centuries, timber was floated down river to Kingston and Garmouth from the forests of Glenmore, Rothiemurchus and Strathspey. Today, water is diverted from the river at various points for power generation.
Crossing was principally by ford or ferry until the 19th century, but one of the oldest bridges is at Garvamore, built in 1731 by General Wade. The remarkable Craigellachie Bridge was the work of Thomas Telford between 1812 and 1814.
In additional to fishing, recreational uses of the river include canoeing and 'white water' rafting.