River Channel Types on the Landscape

Idealized landscape, showing sequence of river channel types. Channel form is related to position in the landscape, which governs the dominant sediment movement processes. Adapted from FISRWG (10/1998), Stream Corridor Restoration

You may have heard the old parable about the five blind men encountering an elephant for the first time.  One of them studies the leg of the elephant, and after a thorough investigation, concludes that an elephant is very much like a tree trunk.  Another one encounters the elephant’s side, and proclaims that an elephant is very much like a big solid wall.  A third investigates elephant’s tail, and becomes convinced that elephants are like stiff rope.  The fourth encounters the trunk, and proclaims the elephant to be akin to a snake.  And the last lands his hands on the tusk, convincing himself that an elephant is like a smooth, stout spear.  The parable is meant to illustrate that we humans make broad conclusions based on incomplete, subjective experience.  Though we may find this parable childish, perhaps even more than a little demeaning, the sad fact is that we approach rivers in much the same way.  Depending on where we encounter the same river, we will find something very different, and create very different beliefs about how rivers behave.

It helps to begin with an idealized landscape such as in the picture above.  The river begins up high, in the mountains, in narrow, steep, rocky headwater streams.  These streams join and flow through gentler valleys, in the mountain foothills.  Eventually, foothills give way to a much broader, flatter landscape, such as a big valley, a prairie, or the coastal plain.  Taken together, all of these parts make up the watershed.  And each part of the watershed has different things going on, which cause the river to have a different aspect or appearance.  Moreover, the appearance of the stream channel, that is, its shape, form or “morphology,” is somewhat predictable based on where in the landscape we are standing.  This is what we will begin to discuss here.

Steep Headwater Streams: the Source Areas

Water flowing down a steep slope has a lot of energy, a lot of ability to erode the soil and even, over long time periods, to erode bedrock.  Streams occurring in these steep, headwater areas carve narrow, deep channels, filled with cobbles and boulders.  All of the smaller material, sand and gravel, is easily picked up and transported downstream.  As the stream channel deepens, over centuries, it makes the surrounding hillsides steeper, making them prone to landslides which can suddenly dump tons of loose sediment into the stream.  Sometimes these landslides flow like a slurry of concrete, for long distances in the headwater channels, scouring out the channel before coming to rest.  The steep streams then proceed to work through these landslide sediments, eroding and washing the finer material downstream.

Because of these steep headwater areas are places where sediments from the mountainside are being eroded and sent downstream, they are often called source areas.  They are just that, the source of most of the sediment that the stream ultimately carries all the way to the ocean.  Streams that appear in these headwater or source areas have a particular look.  They tend to have water cascading over rocks, sometimes in regular sequences of cascades and pools.  The rocks making up the streambed are big, since these are the leftovers after the stream has eroded all of the easily movable stuff.  As such, these rocks don’t move very often, only in the biggest floods or during landslides.  Trees that fall into the stream in the headwater zone often end up spanning the channel like bridges, only touching the water during floods, or jammed into the channel bed while most of the tree remains hung up on the bank.  This large wood can influence the streambed locally where it touches the water, but doesn’t alter the overall shape of the stream channel, and does not tend to move downstream.

Streams in headwater areas are commonly called “cascades,” or “step pool” streams. The dominant sediment movement process in these areas is erosion, and rapid transport of the eroded sediment downstream.

Constrained Valleys with Gentler Slopes: the Transport Zone

As the stream flows downward, into less-steep parts of the mountains, it flows through valleys where the hillsides bordering stream channel are gentler, less steep and thus less unstable.  The stream channel itself is less steep, allowing the smaller cobbles and gravels to be stable on the streambed, among the boulders that still make up the skeleton of the stream channel.  These boulders become organized into clusters or ribs, forming the steps, alternating with pools that may be lined with finer sediment.  And the steps of these “step pool” channels may occur at a regular spacing of 3 to 5 channel widths apart.  Because the stream is not actively deepening itself due to the resistant streambed and banks, and because there is far less sediment sliding in, or being washed in, from the hillsides than there was upstream in the source areas, there is far less erosion going on, overall.  Most of the sediment being moved by the stream is entering upstream, and being transported on through.  Lots of sediment may be moving, but the stream is still too steep to develop deposits of sediment on the streambed except in the sheltered areas behind logs or boulders, and too constrained by the valley walls to form floodplain deposits along its edges.  Because most of the sediment is moving on through, this area is called the transport zone. The dominant sediment-moving process is just that: movement, or transport.

At this point in the watershed, the stream is wide enough that trees which fall into it can lie in the stream channel, altering its shape, creating steps for the water to flow over or pools where the water scours around the log.  Generally, however, trees that fall in the stream in the transport zone will be too big and too well anchored to the bank to be transported downstream.

Commonly, step-pool channels are found in the transport zone.  Or, the stream may take the form of a long, unbroken, gently-sloping chute dotted with scattered rocks and logs that create turbulent “white water.” This is the “plane-bed” channel. However, there are often local areas where the slope is flatter, and the valley wide enough to allow a small floodplain to form from sediment deposited at the edge of the channel.  So, in these areas, the stream can take a different form, where it begins to form curves or meanders, with alternating pools and riffles made of gravel.  This channel form is called pool- riffle.

Flat, Broad Valleys and Plains: the Response Zone

Where the stream emerges from the mountain foothills into a broader, flatter, less confining landscape, it rather suddenly becomes fully alluvial.  We introduced the terms alluvial and colluvial in a previous article.  Alluvial stream channels are lined (streambed and banks) with material that has been transported, and deposited by the water itself.  Non-alluvial, or Colluvial channels, by contrast, are lined by materials that remained as the stream eroded itself in place.  All of the channels in the source areas upstream were colluvial channels.  In the transport zone, the step- pool channels are also colluvial, although they are beginning to have some small alluvial character.  That is, the boulders have been moved around and organized by floods to some degree, and some small accumulations of alluvial sediment are found.  But, these transport zone streams are still constrained or confined by erosion-resistant hillsides. 

As the stream emerges from the foothills, slope decreases abruptly, reducing the energy available to move sediment.  So, the sediment settles out.  And much of it tends to settle out in the area where the stream first emerges from the mountains, forming a broad pile of material called an alluvial fan.  Alluvial fans can be prominent and obvious, but often they are very subtle.  The stream will erode its channel into this broad, indistinct fan deposit, and the fan may appear to be mostly flat.  But the fan is actually a gently convex pile of sediment, a landform slightly higher than the surrounding valley floor. Because it slopes outward, the fan doesn’t confine or constrain where the stream channel flows.  This alluvial fan, at the transition from the mountains to the plains, is usually the most dynamic, changeable part of the entire watershed.  That is because this part of the landscape is actively accumulating sediment, right in the stream channel or next to it.  Eventually, the channel fills with sediment, and the stream moves around that sediment, eroding a new channel which can flow in a different direction, down a different part of the fan.  Trees that fall into the channel here can act as triggers for this kind of channel migration, blocking an existing channel and causing the stream to suddenly form a new one.

This part of the watershed is called the depositional or response zone, because the stream channel is depositing sediment over the long-term, and the stream channel is very sensitive to change.  It responds quite readily to changes in the amount of sediment moving in, to blockage by trees, or to the energy of a large flood or pulse of sediment from upstream.

The stream channel in these response areas often will not be one channel, but multiple channels that split and then come together, separated by islands of floodplain.  This type of stream channel is called “Island-braided” or “anastomosing.”  Anastomosing means splitting and coming together, a medical term used to describe networks of blood vessels.

Further downstream, beyond the transition from mountain to valley that the alluvial fan occupies, the stream flows through a true floodplain.  The floodplain is the relatively flat area next to the stream channel, which is inundated regularly, and is built of alluvial materials transported by the stream itself.  In a floodplain, it is typical for the stream to develop a form that we call “pool-riffle.”  The stream will curve or meander back and forth, forming a deep pool on the outside of each bend, and a gravel or sand bar on the inside of the bend.  The channel between bends in the river becomes shallow and turbulent, and is called a riffle.  Upstream, the alluvial stream channel is made of gravel.  Downstream, as it continues to flatten, everything is made of sand.

In a floodplain, there is very gradual accumulation of fine sediment (sand, silt and clay) from overbank floodwaters. This is what builds the floodplain soils. But in the stream channel itself, there is a rough balance, or equilibrium, between sediment being deposited (e.g. on gravel bars) and sediment being eroded (e.g. on the outside bends of meanders). Scientists call these areas “exchange reaches,” because the dominant sediment process is exchange of one sediment grain for another, as balanced deposition and erosion occurs.

When trees fall into the river in this part of the watershed, they have a profound effect on the shape of the channel and its change over time.  That is because the river is big enough to move these trees around, causing them to accumulate into logjams.  Logjams, in turn, can divert the river channel, and since the floodplain is made of alluvial materials that the river is capable of moving, trees or large wood become a major player in the process of channel migration.  Channel migration is a normal feature of alluvial, floodplain rivers.  Even in the absence of logjam formation, the pool- riffle channel will erode the outer edge of each meander bend, causing the channel location to migrate, gradually, over time.  Logs and logjams simply speed up that process, and make it occur in sudden, unpredictable events.

As the stream approaches its end, which could be a bay, a lake or the ocean, it becomes very flat.  Here, the channel splits and flows in multiple directions, through what are known as distributary channels.  Sediment deposition creates a landform called a delta. 

What we have described above is an idealized version of nature.  Real rivers are usually more complex than this, but the idea is that the type of stream channel that we see, is predictable based on where we are in the watershed.  And that is because the processes at work creating the stream channel, processes such as erosion, transport, and deposition of sediment, input of large wood in the form of trees, and the movement and accumulation of this large wood, are dependent on location within the watershed, occur in predicable places.

References:

FISRWG (10/1998). Stream Corridor Restoration: Principles, Processes, and Practices. By the Federal Interagency Stream Restoration Working Group (FISRWG)(15 Federal agencies of the US gov't). GPO Item No. 0120-A; SuDocs No. A 57.6/2:EN 3/PT.653. ISBN-0-934213-59-3.