3.1.4.3, 3.1.4.4 - Meltwater and fluvioglacial landscapes
Meltwater transfers both water and sediment through and beyond glaciers. Its variable discharge produces distinctive channels and deposits that differ from direct ice deposition.
Meltwater erosion and transport
Meltwater flows on, within, beneath and beyond glacier ice. Surface melting supplies seasonal flows, while geothermal heat or release from stored water can also feed drainage. Pressurised subglacial water follows hydraulic gradients influenced by ice pressure as well as bed elevation; it need not follow the simplest downhill route suggested by an ice-free surface map.
Fast water erodes through hydraulic action and abrasion by carried sediment. Sand and gravel may move as bedload by rolling and bouncing, finer particles in suspension and dissolved material in solution. Greater discharge and velocity can increase transport capacity, but sediment supply and channel form also matter. Daily and seasonal changes, or sudden outburst floods, can rearrange routes and sediment rapidly.
Meltwater channels can be cut alongside or beneath former ice. After retreat some appear oversized or dry because the meltwater supply and former ice control have gone. Their position relative to moraines and valley slopes helps distinguish a former marginal drainage route from an ordinary modern stream. A channel alone cannot establish its discharge, date or whether all its erosion was glacial meltwater.
Deposition, sorting and outwash plains
Water deposits sediment when its capacity to carry it falls, for example as a stream spreads, slows or loses discharge. Coarser particles generally settle sooner, while finer material travels farther under otherwise similar conditions. Repeated flows lay down layers, making fluvioglacial deposits commonly sorted and stratified compared with direct ice till. Later disturbance can blur this distinction, so several sediment properties should support identification.
Beyond an ice margin, sediment-rich meltwater can divide into braided channels across an outwash plain, also called a sandur. Deposited bars divert flow into new channels; changing discharge remobilises them. The plain generally slopes away from the glacier and contains spatially varying gravel, sand and finer sediment. It is an active drainage and deposition system rather than one uniform sheet of sand.
Skeiðarársandur in southeast Iceland is a large active outwash landscape associated with Vatnajökull drainage. The national park describes shifting glacial river routes, including diversion of Skeiðará in 2009. This illustrates changing connections between ice, water and sediment; it should not be simplified to glacier ice depositing the entire plain directly. Outburst floods can erode older surfaces as well as add sediment, so the assemblage preserves repeated events.
Continue this lesson.
You’re previewing NovaLearn without an account. Create a free account to read the rest.
Create free account