Today it provides a water supply to several surrounding cities, and its steep-sided valley is an important communications corridor through the uplands of the Peak District.

[3][4] Derwent is derived from Brittonic river name *Deruentiū, Latinised as Deruentiō, meaning "(belonging/pertaining to the) forest of oak trees"; the old river name survived in medieval Welsh poetry, such as Peis Dinogat ("Dinogad's Smock") attached to the larger poem Y Gododdin, as Derwennydd.

[5][6][7][8] The River Derwent rises at Swains Greave (590 metres (1,940 ft) above sea level) on the eastern flank of Bleaklow, opposite Howden Moors, and some 6 miles (9.7 km) east of Glossop.

Below this confluence, it flows through Hathersage, Grindleford, Calver and Baslow, and through the estate of Chatsworth House, before it is joined by the River Wye at Rowsley.

After passing through Darley Dale, the Derwent reaches Matlock, where, at an oxbow, it collects the Bentley Brook.

It then flows past the villages of Matlock Bath, Cromford, Whatstandwell, and Ambergate, where it is joined by the River Amber.

[17] The power of the Derwent is still harnessed at a number of these historic mill sites, producing hydro-electricity from turbines instead of driving mill wheels, with a recent development being the construction of a hydro-electric station at Longbridge weir, adjacent to the Riverside Gardens in Derby.

[18] Howden and Derwent Reservoirs in the upper valley were both completed in 1916 to supply the cities of Sheffield, Nottingham, Derby, and Leicester.

Treated water from these reservoirs flows down the 28-mile (45 km) Derwent Valley Aqueduct parallel to the river.

When flows are low, water is released back into the river via the same 6.5-mile (10.5 km) route of tunnels and aqueducts, thus allowing greater abstraction rates downstream at Little Eaton in the drier summer months.

[24] The pureness of the river has also been commented on by Charles Cotton – "Piscator" in Izaac Walton's Compleat Angler.