The dynamic delta of Urquhart Bay Loch Ness
The rivers Enrick and Coiltie meander towards their mouths in Loch Ness, altering their deltas through erosion and sedimentation. These river systems are very dynamic; flooding further alters the estuaries. The changing patterns of the river channels result in gravel banks and redundant river channels with still pools. Geology in miniature!
Urquhart Bay Wood, known locally as "The Cover", is one of the few intact alluvial forests in the UK; a habitat considered rare throughout Europe. It is a wet forest that is hydrologically connected to a river system. The forest lies in a flat, low-lying area of alluvial sand and gravel deposits.
Please explore the meanders and deltas, make your own observations an answer the following questions:
- Describe the meanders and briefly explain in your own words how they came into being and how they are likely to develop further. How big is the height difference between the impact slope and the sliding slope?
- Please describe the area of the delta. Why did it come into being in this form?
- What different forms of banks can you see? In which places are which grain sizes of sediments present? What explanation do you have for this?
- Please take a photo of yourself or a personal item at the confluence of the Coiltie and the Enrick or the mouth into Loch Ness and attach it to your log.
River Delta
A delta estuary is formed when the river's flow slows to virtually zero in the estuary area, so that it deposits most of the material it had previously carried as sediment.
With sea or lake levels remaining largely constant, the river shifts its mouth further and further out to sea or lake over centuries and millennia by continuously dumping its sediment load, and the former mouth area becomes silted up. The river's own deposits increasingly block its path to the sea or lake, so that the main stream forks several times. This creates the characteristic delta shape that can be seen in maps and satellite images of these special river mouths. A prerequisite for the formation of delta mouths is a low tidal range in the mouth area, as otherwise tidal currents prevent the undisturbed build-up of an estuary fan.
Meanders
Meanders are curved loops of flowing water that change dynamically over long periods of time. The course of a stream or river is determined primarily by the flow speed, the sediment load and the nature of its subsoil.
The sediment load is the amount of solid matter that is mobilized in a catchment area and transported away via the river system. In a river, the gradient tends to decrease from the source in the mountains to the mouth, and with it the flow speed. In the upper reaches, such a stream or river may initially carry large boulders and gravel, but in the mouth area it often only carries fine sand or tiny clay particles (suspended load). All the material that can no longer be transported due to the decrease in flow speed is deposited in the riverbed on the way from the source to the mouth (sedimentation).
The deepening of the riverbed that accompanies the erosion of a river is called deep erosion, and its widening to the side is called lateral erosion. In fast-flowing mountain streams with a steep gradient, deep erosion predominates. In slow-flowing streams or rivers with a low gradient, lateral erosion predominates, which then usually leads to the formation of meanders.
Bars
A bar in a river is a raised area of sediment (such as sand or gravel) that has been deposited by the current. Bar types include mid-channel bars, point bars (common in meandering rivers), and mouth bars (common in river deltas). The locations of bars are determined by the geometry of the river and the flow through it. Bars reflect sediment supply conditions and can show where the sediment supply rate is greater than the transport capacity.
A point bar is an area of deposit typically found in meandering rivers. Point bars form on the inside of meander bends in meandering rivers. As the current flows around the inside of the river bend, the water slows down due to the shallow flow and low shear stresses reduce the amount of material that can be transported there. Point bars are usually crescent-shaped and are found on the inside curve of the river bend. The excess material falls out of transport and over time forms a point bar. Point bars are typically found in the slowest, shallowest parts of rivers and streams and often run parallel to the shore and occupy the area farthest from the thalweg, on the outside curve of the river bend in a meandering river. Here, at the deepest and fastest part of the stream, is the break-off bank, the area of a meandering riverbed that is continuously subject to erosion.The faster the water in a riverbed, the better it can pick up larger amounts of sediment and larger pieces of sediment, increasing the river's bedload. Over a long enough period of time, the combination of deposition along point banks and erosion along break-off banks can lead to the formation of an oxbow lake.
A mouth bar is a raised area of sediment typically found in a river delta, located at the mouth of a river where the river flows into the sea or a lake / loch. Sediment is carried by the river and deposited in the middle of the channel at the river mouth. This happens because the river widens at the mouth, the current slows and sediment settles and is deposited. After the initial formation of a river mouth bar, these tend to advance. This is caused by the pressure from the flow on the upstream face of the bar. This pressure creates erosion on that face of the bar, allowing the flow to transport this sediment over or around, and re-deposit it farther downstream, closer to the mouth. River mouth bars stagnate, or cease to prograde when the water depth above the flow is shallow enough to create a pressure on the upstream side of the bar strong enough to force the flow around the deposit rather than over the top of the bar. This divergent channel flow around either side of the sediment deposit continuously transports sediment, which over time is deposited on either side of this original mid channel deposit. As more and more sediment accumulates across the mouth of the river, it builds up to eventually create a bar that has the potential to extend the entire length of the river mouth and block the flow.
Please send me the answers via the contact form of my GC profile or the message function. Afterwards you can log directly, you do not have to wait for a log-release. Please don't forget to upload your picture.