glacial rivers Glacial rivers are distinguished by four main characteristics:(1) their regular seasonal and diurnal cyclical fluctuations;(2) their liability to numerous irregular and catastrophic flood events associated with sudden drainage of ice-dammed lakes;(3) their high sediment loads, especially of suspended sediment; and(4) their tendency to form extensive and unstable braided channel systems.

Cyclical fluctuations of melt-water run-off

Most glacial rivers are characterized by strong seasonal variations in melt-water discharge (except in equatorial glacierized basins) where discharge fluctuations closely follow the annual temperature curve. Thus, during winter there is little melt-water flow, for the catchment remains frozen and the only source of melting is from subglacial geothermal heat flux. Spring melt-water flows begin with the break-up of river ice followed in early midsummer by a rapid rise in snow-melt run-off. Further seasonal warming leads to melting of glacier ice, which generates the maximum melt-season run-off into glacial rivers because flows are now composed both of melt-water derived from ablation (melting) and rainfall inputs, and from stored water, all of which run off rapidly through fully expanded englacial routeways. By late summer, supplies of stored water are exhausted, and, as temperatures begin to decrease, discharge declines in the form of a recession curve. Finally, as autumn comes to an end, the winter freeze-back begins and flows approach their seasonal minimum.

Most glaciers also exhibit a strong diurnal cycle in run-off, reflecting diurnal temperature fluctuations, and thereby generating the equivalent of a complete flood cycle every 24 hours. The timing of the peak daily flow largely depends on the transit time for water to flow through the glacier system from different parts of the ablation zone. Hence, peak flows occur earlier in the day towards the end of the melt-season, when the tunnel systems are most fully developed, and in rivers draining small glaciers.

Irregular and catastrophic run-off events

Superimposed on the regular seasonal and diurnal cycles of run-off are two other populations of flood events. First, floods generated by summer or autumn rainstorm events which, when combined with relatively warm conditions, may form the largest floods of the melt season. Secondly, floods generated by sudden failure of an ice dam which is ponding up melt waters in an ice-marginal lake. These floods are known as ‘glacier burst’ or ‘jökulhlaup’ floods in which discharges commonly exceed ‘normal’ ablation-related flows by up to several orders of magnitude (see jökulhlaups). For example, Michael Church, working in Baffin Island, found that jökulhlaup flows from sudden lake drainage reach c. 200 m³ s⁻¹, compared with more normal diurnal peak flows of c.20 m³s⁻¹, and accounted for over 10 per cent of total melt-season run-off. By contrast, Helgi Björnsson, studying jökulhlaups from the Grimsvötn area of the Vatnajökull ice cap in southern Iceland, which result from subglacial geothermal activity, found that they can reach magnitudes of 40 000 m³ s⁻¹ (e.g. in 1922), compared with more normal run-off flows of c.400 m³s⁻¹. Some of the largest floods on Earth have been jökulhlaups. For example, during deglaciation of the Pleistocene ice sheets in North America, ‘Glacial Lake Missoula’, which impounded up to 2500 km³ of water at the edge of the Cordilleran ice sheet in Washington State, drained catastrophically through failure of an ice dam to generate peak river flows of 21 × 10⁶ m³ s⁻¹. Similarly cataclysmic floods occurred in the Altay Mountains in Siberia, where Late Pleistocene glacial lakes drained catastrophically to generate peak flows of 18 million m³ s⁻¹.

High sediment loads

Glacial rivers are characterized by extremely high sediment loads. Suspended sediment loads generally exceed 1000 p.p.m. (parts per million) and 400 metric tons km⁻² of catchment area per annum. However, the actual amounts also depend on the resistance of the bedrock to glacial and glacio-fluvial erosion. In ancient crystalline shield areas, for example, suspended loads of glacial rivers are significantly lower than those of young, mountainous volcanic regions. For example, Church's measurements of suspended sediment concentrations of melt waters in Baffin Island indicated peak amounts of 1060 p.p.m., compared with 39 000 p.p.m. recorded by Kazimier Klimek from melt waters on Skeidararsandur in southern Iceland. Such high volumes of sediment are derived both from (a) a variety of sediment transport zones within the glacier and (b) non-glacial sources, such as mountain slopes, fans, avalanches, and rain- and snow-fed tributary streams. One study from the Hilda Glacier in Alberta, Canadian Rockies, for example, estimated that 6 per cent of suspended loads were derived from supraglacial and englacial debris, 47 per cent from subglacial sources, and a further 47 per cent from materials lining the banks of the proglacial rivers. Sediment sources are thus highly variable over space and time, the former reflecting the geographical distribution of sediment pathways to the proglacial environment, and the latter reflecting seasonal, diurnal, and episodic variations in run-off and availability of sediments. For example, jökulhlaup flows have been found to account for between 25 and 75 per cent of the total annual sediment transport from glacial catchments. Nummedal and his colleagues have estimated that a 2-week jökulhlaup on Skeidarasandur could move as much sand as would otherwise be transported in 70–80 years of normal melt-water flows.

Braided channel patterns

Proglacial river systems are widely characterized by the development of braided channel patterns. These result from the high degree of channel instability associated with numerous and repeated flood cycles that act to weaken the non-cohesive channel bank deposits. These are made highly erodible by the absence of binding vegetation and the paucity of silts and clays, which are washed away by daily high flows. Channel widening through repeated bank collapse leads to shallowing of the flow and erosion of meander bends, so that many glacial rivers characteristically form shallow channels of low sinuosity. High volumes of sediment laden melt waters flowing through shallow channels can lead to large-scale aggradation. In mountainous catchments aggradation is confined between valley walls to form a ‘valley train’ or ‘valley sandur’, while river systems that are unconfined can form vast ‘outwash’ plains or sandurs. Modern sandur plains are located typically in piedmont zones, especially those which extend seawards from young glaciated mountain ranges on to coastal plains, such as those of southern Iceland, south-western Alaska, and western Greenland. Pleistocene (ice-age) sandurs were more extensive including, for example, the Canterbury Plains of South Island, New Zealand; the Patagonian plains of southern Argentina; and those bounding much of the North American and Eurasian ice sheets. Huge glacial rivers crossed these plains, forming the ancestors of such major drainage routeways as the Mississippi and the St Lawrence, the Rhine and the Danube, the Ob and the Yenisei, many subject to catastrophic jökulhlaup flooding and drainage diversions during each episode of glacial fluctuation and ice-sheet decay.

Judith Maizels

Bibliography

Baker, V. R. (1973) Paleohydrology and sedimentology of Lake Missoula flooding in Eastern Washington. Geological Society of America Special Paper, 144.
Church, M. and and Gilbert, R. (1975) Proglacial fluvial and lacustrine environments. In Jopling, A. V. and McDonald, B. C. (eds) Glaciofluvial and glaciolacustrine sedimentation, pp. 22–100. Society of Economic Paleontologists and Mineralogists Special Publication No. 23, Tulsa, Oklahoma.
Maizels, J. K. (1995) Sediments and landforms of modern proglacial terrestrial environments. In Menzies, J. (ed.) Modern glacial environments. processes, dynamics and sediments, pp. 365–416. Butterworth-Heinemann, Oxford.