Where I live in the Cheviots, water moves in one direction: downwards. It falls out of the skies, soaks through the heather and bog to join a head of groundwater hiding in the soil and stone, or just runs off the surface downhill. It finds springs and streams, and with the steep slopes it encounters moves along at a jolly tinkling pace.

It is hard to imagine when watching the clear water of the College Burn flowing gently over boulders (made of andesite) and sparkling in the sunlight just how destructive this river can be when in spate. However the size of the boulders in the riverbed are a clue. To push boulders along weighing upwards of 100kg takes a great deal of force. This extreme of sediment transport is even more blatant in major mountain ranges such as in the Himalayas where meltwater adds to the volume and pace of water arriving in the rivers.

Change is of the essence for streams and rivers. These fluvial channels are continually moving sedimentary and organic material downstream along with their water, and in the process change the size, shape and path of their channels. The rate of flow changes too. As well as the variation in flow rate from season-to-season and from cloudburst to sunshine, the changes in dimension of the channel will throttle or enhance the flow of water. Just a few moments on a sandy beach with a rivulet running across it will show the way that sediment moves along the bed of the channel changing the shape, direction and flow of the channel as it goes.

This change of flow is not just about the sedimentary material the water carries but what it does to the channel in which it flows.  Rivers incise, eroding away their banks in v-shaped notches in the mountains and into ever-broadening valleys further down-stream; but they not only erode, they also deposit. The fans where a river splays out from the base of a mountain, the river terraces in the floodplain of a river valley, and the broad deltas at a river’s mouth are all places where piles of sediment are accumulating. What is it that allows the river to both give and to take?

The physics of flow is complex, as I discovered during my doctoral work, when I investigated the impact of magma flows on heat transfer to surrounding rocks, but that’s another story for another blog! However, there are some neat ways of showing how flow-rate and sediments interact with each other in rivers.

In the 1930s Filip Hjulström took a daily sample from the River Fyris on his way to the department of geography at Uppsala University where he was studying for his doctorate. The detailed analysis of the sediment contained in these samples provided the first quantitative analysis of the transport, erosion and deposition of sediment in rivers. This was neatly summed up in his doctoral thesis in what became known as the Hjulström diagram. Hjulström went on to set up a department of hydrology at Uppsala University and one of his students Äka Sundborg further refined this diagram to produce what is known as the Hjulström-Sundborg Diagram. This diagram makes it easy to see the way in which sediment is either eroded or deposited depending on the flow rate and the particle size and at what flow rates a given particle size will be transported. 

For sandstones, you can see that there is a linear relationship between the size of particle being deposited and the flow rate; as flow slows down, progressively smaller particles maybe deposited (and vice versa). By understanding this, it is easy to see how small fluctuations in the flow rate will create changes in the particle size being deposited. This results in the sort of banding which can be seen in the Hadrian’s Wall mystery rock number four, from Haltwhistle Burn. In this case the change of particle size is emphasised by changes in the mineral types as the particle size changes.

The College Burn joins the River Bowmont at Westnewton and flows east down as the River Glen meandering out onto the Millfield plain (the remains of a post-glacial lake) where it joins the River Till near to Doddington (the home of ice-cream and cheese!). The River Till flows downstream at right angles to the River Glen, running to the north and west parallel to the coast. It is flanked by the Cheviot volcano to the west and the high land of the Fell Sandstones to the east. After transiting the Millfield Plain, it flows through a mini-gorge for another 5 miles as the crow flies.

Just after passing Twizel Castle it meets the NE flowing River Tweed and turns another right angle. A few miles further on down the Tweed at St Thomas’ Island near to Horncliffe it starts to feel the upcoming tendrils of the tide. 

This is an important marker with a sedimentologist’s hat on, as flow is no longer just down-gradient. Here the down-slope force of the Earth’s gravity starts to be matched periodically with the gravity of our Moon’s influence on the sea. The river’s flow weakens, stills and finally turns in the face of the rising tide and then repeats in reverse. This regular change in both direction and pace leaves its mark in the sediments. New patterns can be found in the layers of rock laid down as we head into the river delta with its ebb and flow and the taste of salt.

There is more to say about the patterns found in fluvial sediments formed both with unidirectional flow and bidirectional flow. That is material for another blog, so I will leave you with our drops of Cheviot water floating out to sea as it rounds the last bend in the river to meet the sea between Berwick and Tweedmouth.

@Northumbrianman

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