Erosion by water occurs when water is flowing across a surface and the flow is capable of transporting more sediment than is currently moving as bedload. This is called the sediment transport “capacity”. A certain number of grains of a certain size can be picked up by the Bernouli effect for a given flow. If there are too many grains, they start colliding and the characteristics of sediment transport change. Grains are directed back toward the bed and up into the flow. Eventually, more go back to the bed and are deposited, leaving fewer grains in the flow even at high flow speeds because there are more grains than the transport capacity of the flow.* In contrast, if there is a shortage of grains of a size that can be moved by the flow, e.g. the flow is moving all of grains present, any new grains will be eroded off the bed as soon as they are available. The flow then has excess transport capacity.
* Think about dumping a truck load of fine sand into a fast moving river, it takes time to move all that sediment even if the flow speed is theoretically fast enough to erode fine sand.
One of the most common times for a flow to have excess transport capacity is when the flow is speeding up. We know from the Hjulstrom diagram that faster flows transport larger grains. They can also transport more grains. Thus, water flowing downhill commonly speeds up, has excess capacity and erodes sediment. When it slows down, sediment is deposited. In floods, the water speeds up, erodes sediment, and transports it. As the flood ends, the water slows down and deposits the excess sediment. In general, erosion occurs when flows are speeding up or when they go from an environment with low sediment (e.g. a dam spillway) to an environment with more sediment (e.g. a river bed).
Rivers are responsible for most sediment transport from mountains to lowlands and the oceans. They do the most to even out the topography that tectonic processes create. Rivers consist of channel, bank and overbank or floodplain deposits. Most of the sediment and many river characteristics are controlled by the highest common flow speeds.
River Types -
Straight (rare, except for ones humans have modified)
Meandering (high sinuosity)
Braided (many branches within a channel)
Anastomosing (rivers with branching and merging channels)
The form of the river is controlled by the gradient of the river bed (steep = braided, gently dipping = meandering), local vegetation that stabilizes banks and limits the number of channels, sediment grain size, particularly the ratio of suspended versus bedload sediment, and sediment volume. A high bedload gives rise to abundant bars, which promotes formation of braided rivers.
Braided rivers develop when the proportion of bed load sediment is high, which produces abundant bedforms and promotes the development of bars, and thus, the braided character of the river. The sediment is commonly coarse, which requires fast flow and steep gradients for the sediment to be transported. Much of the geometry of braided rivers is shaped by the highest flows, e.g. spring floods, when the bars are covered in water. Many braided rivers have exposed bar tops for much of the year.
Flow speeds and transport capacity vary dramatically within a braided river. Friction with the riverbed tends to slow down the flow, particularly where the flow is shallow. Thus, the Reynolds number in shallow areas is relatively low (but still high enough that the flow is turbulent) and the transport capacity is low. In contrast, the transport capacity and Reynolds number are much higher in the deeper middles of channels in the river. Thus, the coarsest sediment is transported here, whereas finer sediment gets deposited in shallow areas. Also, bars block the flow on the upstream sides, and like dunes, the upstream sides tend to erode. Areas of low flow and eddies form on the downstream sides of bars, and they are usually sites of net deposition. Thus, bars migrate downstream through time. If we summarize the processes:
- The coarsest sediment is transported in the middle of the flow where the Reynolds number is highest.
- Bars are eroded upstream where the bars deflect the flow. Sediment is deposited on downstream side of bars and some on the flanks of bars where flow is slower, particularly on the insides of bends.
- Secondary bedforms, i.e. planar beds, dunes, and ripples, form as a result of sediment transport on the bars and in the channels.
Sedimentary structures include:
- trough x-bedding in channels, due to the migration of irregular dunes
- coarsest sediment may be lower flat laminated if flow speeds are not fast enough to form coarse grained dunes
- sediment on the edges of bars fines upward because the flow is shallower and slower, e.g. has a lower Reynolds number. Sedimentary structures can include anything from upper flat to ripple laminations.
Braided River Facies
Channels migrate back and forth leaving a sheet of sand with abundant cross stratification. These sheets of sand tend to fine upward. General characteristics of braided river deposits include:
- Scoured surface at the base of a channel
- Gravel lag at base of channel
- Trough x-bedded sands deposited just off the center of channels
- Occasional tabular x-stratification from migrating bars
- Sand deposited at slower speeds (ripple cross lamination possible)
- Overbank deposits from floods mostly composed of sand and silt, with some mud
The large scale geometry of the deposits includes sheets of sand with various grain sizes representing bar migration separated by floodplain deposits.
Meandering rivers have a low gradient and thus slower flow, and usually have a high proportion of suspended sediment relative to the amount of bedload. A meandering river channel has curves that meander back and forth on a gently sloping floodplain. The flow speed in the channel varies with the geometry of the meanders. Water has to travel faster on the outside of bends than on the insides of bends. We know from the relationships between Reynolds number and bed shear stress that higher flow speeds mean that more and coarser sediment can be transported at higher flow speeds. Thus, we can predict that:
- there is more erosion on the outsides of bends
- the sediment moving near the outsides of bends and in the deepest parts should include the coarsest sediment available
- sediment will accumulate on the insides of bend and this sediment will be finer grained.
If we look at a channel in cross section, it is asymmetric, representing the sites of erosion and deposition. Variation in flow speed also produce different sedimentary structures. Upper planar lamination and dune cross stratification are common where Re is highest, and ripple cross lamination is common where Re is lower.
The main parts of the channel include eroding bank, the thalweg (the deepest point of the flow) and the point bar (on the inside of the bend where most sediment is accumulating). As the channel migrates due to erosion and deposition, a distinctive suite of sedimentary structures accumulate. The deepest part is coarser and has upper planar lamination or dune cross stratification. This is overlain by finer sediment with current ripple lamination.
As meandering rivers migrate, the meanders tend to increase. Eventually, the channel forms almost a circle, and the meander gets cut off, often during a flood. This straightens the channel temporarily and produces an ox bow lake in the abandoned meander. The lake accumulates mud and organic matter.
Watch this cartoon of a meander migration in France: http://faculty.gg.uwyo.edu/heller/SedMovs/Meander_Alliers.htm
Levees and Floodplains - When a river floods, it goes from a confined flow in the channel which is very rapid to a widespread flow across the floodplain. It slows down very quickly and the water becomes shallower, both of which cause a decrease in Re. Thus, the water can not transport as much sediment on the floodplain as it does in the channel. Thus, finer sands that may be in suspension during a flood are transported as bedload or rapidly deposited once the river tops its banks. This produces levees. The finer silts and especially clays remain in suspension much longer and settle out on the floodplain as the flood waters dry up.
Watch this model of a meandering river flood: http://faculty.gg.uwyo.edu/heller/SedMovs/RhineFlood.htm
Over time, the levees build up and provide a higher bank for the channel than the level of the floodplain. Thus, the channel bottom can aggrade (fill in) until the bottom of the channel is as high or higher than the floodplain. When the next flood comes along, the river avulses and does not go back into its old channel which is higher than a new one on the floodplain. This results in the downstream part of the channel being completely abandoned.
Meandering River Channel Facies
- Scoured base of flow
- Lag deposit with mud rip-up clasts and the coarsest grains being transported
- Fining upward sands with trough cross stratification
- Rippled sands
- Sigmoidal cross stratification from migrating point bars
- Fine sand with climbing ripples
- Mudstone/shale with mud cracks
- Root casts
Ox Bow Lake Facies
- Mudstone/shale without mud cracks
- Organic-rich deposits, including coal
- Anoxic water indicators (especially in fossils and absence of trace fossils)
Differences between braided and meandering river deposits
- Braided river deposits are commonly coarser grained
- Meandering rivers contain abundant suspended sediment, which is deposited in ox bow lakes and on floodplains.
- Overbank deposits are better developed and finer grained in meandering river systems.
- Bar migration is much more regular in direction in meandering rivers because there is a well defined, single thalweg towards which the bars migrate. In contrast, braided river bar migration occurs in multiple directions. Thus, meandering rivers produce a more regular geometry of tabular cross bedding, when preserved.
General Characteristics of Fluvial Sediments
- On a large scale, river deposits consist of sheets and lenses of sand deposited in channels associated with flat laminated shales and silts with rare rippled sand beds deposited on floodplains.
- Fining upward sequences of beds in the sands with sedimentary structures that indicate decreasing flow speeds.
- Abundant cross stratification in well sorted sands, particularly trough cross stratification.
- Cut banks at the edges of channels - these are good indicators of a migrating river channel, but can be hard to see in outcrop since they are rarely preserved
- Soil development in associated shales deposited in the floodplain environment.
Look at pictures of fluvial rocks at http://mygeologypage.ucdavis.edu/sumner/gel109/SedStructures/Fluvial.html