With reference to one or more areas you have studied, describe and explain how past glaciation has had a large influence on the present-day drainage.
The Pleistocene glaciations had a large effect on the present drainage patterns. Ice in glaciers and ice sheets can block channels or carve out new ones. Ice can dam up lakes and carry large amounts of sediment. Glacial landforms can also act as barriers or channels long after the ice has retreated.
One of the simplest ways in which glaciation can alter drainage is if a glacier obstructs the flow of a river channel. In some cases the river will simply flow by another route. A typical example is that of the Thames.
In the lower and middle Pleistocene, the Thames used to follow a northeasterly course through North London. The first phase of the Anglian glaciation formed the 'St Alban's lobe', blocking the course of the Thames and forcing it to take a more southerly route, before turning north and rejoining its original course at Ware. A second ice advance formed the Finchley lobe, blocking the second course and forcing the Thames into its current southerly position.
Sometimes the river cannot flow elsewhere so an ice-dammed lake is formed. Ice-dammed lakes are defined as 'lakes impounded against the margins of a glacier'. They tend to occur in three ways:
There are many examples of ice-dammed lakes in the UK and elsewhere. In Britain, a huge lake (named Lake Harrison) used to cover much of the Midlands around what is now Warwick, Birmingham and Leicester. It was formed when ice from Wales and the North blocked the Avon, Tame and Soar valleys during the Woolstonian glaciation. This created Lake Harrison between the ice-front and the Jurassic escarpment.
Over time, ice-dammed lakes inevitably become fuller and eventually they flow over a 'col' or low point into another valley. This happened to Lake Harrison, which overflowed to the SW and SE. To the SE there was an overflow across the Fenney Compton Gap through the Cherwell Valley and into the Thames. This has now been abandoned. However the SW overflow became the new course of the River Avon so nowadays it flows to the SW and out into the Severn rather than following its pre-glacial NE direction.
The Severn itself is another good example. The pre-glacial Severn flowed North into the Dee estuary but the Irish Sea ice sheet blocked its course during glaciation causing Lake Lapworth to form. It then overflowed to the south creating the spectacular Ironbridge Gorge (see right).
Another classic example of ice-dammed lakes and overflow over cols is the 'Parallel roads of Glen Roy'. Here, the slow retreat of the ice gradually uncovered a series of cols, which allowed the lake levels to fall. Originally the highest lake was 355m above sea level. Nowadays, there are misfit rivers on the higher levels, while the main lake is 258m high.
Misfit streams or rivers are watercourses where the stream or river is far too small to have created the associated valley. For example the huge U-shaped valley of Nant Ffrancon in Snowdonia has only a small stream running through it. A valley glacier carved out the original valley, but after glacial retreat, the 'fossil drainage' remains.
Other glacial landforms can also affect the flow of rivers. For example, the Ribble Valley around Hellifield in Yorkshire was diverted by drumlin swarms to a more southerly route.
Nowadays the River Aire carries on the course of the Ribble beyond the drumlin swarms.
The USA's drainage pattern was markedly affected by glaciation.
Pre-glacial drainage pattern
Post-glacial drainage pattern
Prior to the Pleistocene glaciation, the Great Lakes (Superior, Michigan, Huron, Erie, and Ontario) on the border between the USA and Canada did not exist. The area was flat lowland, underlain by soft sedimentary rocks, with stream valleys draining to the north. Pleistocene lobes of the North American ice sheet pushed southward into the pre-existing stream valleys, deepening them by glacial erosion. These valleys became the Great Lakes basins. Upon retreat of the ice these basins filled with meltwater to become the Great Lakes.
A final way in which glaciation can affect drainage patterns is during glacial retreat, when the huge volumes of meltwater can erode large, steep-sided meltwater channels, such as the Gwaun valley in Snowdonia. The valley now contains streams flowing in both directions, since although the pressure on the meltwater allowed it to flow uphill, the present misfit streams do not have enough energy and hence flow in both directions from the high point in the middle of the valley.
Glaciation has had a major effect on drainage pattern throughout the UK and elsewhere. Many of the UK's major rivers had their courses radically altered by glaciation, including the Thames, Avon, Severn, Esk, Derwent and Dee. The huge erosive power of ice means new valleys can be carved in a fraction of the time water would take. In our time, these large U-shaped valleys are often filled by misfit streams. Ice can also act to block rivers, creating huge lakes like Lake Harrison. These overflow creating dramatic gorges like Ironbridge. Thirdly glacial deposition can act as a barrier to rivers, diverting them like the Ribble. Finally glacial meltwater channels can be created as the ice retreats, with now only misfit streams remaining. After geology, glaciation is probably the single most important factor in determining drainage patterns, and as such, its importance should not be underestimated.