3.1. How a Stream Works

The basic structure of a stream influences its health and ability to provide habitat. Stream structure is a balance of its soils, water table levels, and land elevations, creating a habitat that is unique with each stream. Understanding this balance is important: enhancement of habitat often corrects problems that occur when people tamper with its balance.

The structure of a stream is defined by:
- Water flow features (the stream's pools, riffles or other specialized sections)
- Channel (the stream's width and shape)

Let's look at each one of these so we understand what they mean.

 


Water flow Features

The flow of a stream can be broken down into four main areas

- Riffles

- Pools

- Runs

- Flats

3.1.1 Waterflow Features

Riffles
These are the swift, shallow portions of streams or rivers where the water surface is broken. Often rocks protrude through the water surface. The stream bottom is usually gravel and rock. Riffles are important areas for two major reasons. Most of the food supply for salmon or trout is produced on them and young salmon will establish feeding territories here during the summer. Salmon that are laying eggs (spawning) select areas where water is seeping through the gravel. These areas are usually found at the tail of a pool or at the head of a riffle.

Runs
These are swift, deep portions of streams or rivers. Although water flow in a run may be as swift as in a riffle, the water is deeper. Runs can range in depths (depending on stream size) from 20 cm to 2 meters (8 in. to 6.6 ft.) deep. The stream bottom is usually rock and boulder.

Pools
These are slow, deep portions of rivers and streams (in proportion to the watercourse's size). Pools can be of various depths, but are the deepest areas of a stream. The bottom can be gravel, rock, boulder, silt or log-strewn. Pools provide living areas for a variety of fish. These slow, deep areas are a refuge in the winter for many species of fish, and provide ideal cover for some of the largest trout. Pools are critical habitat for salmon during low flow periods in the summer and winter, for the migration of adults as holding and resting areas, for spawning adults holding areas and as winter habitat for pre-smolts. During normal flows pools are dominated by trout, and the size of the pool and available instream cover, are the major determining factors on how many, and how large a trout, can live there. The largest numbers of speckled and brown trout are found in pools.

Flats
These are the shallow, slow portions of streams, usually located at a point where the watercourse widens. A flat can be as shallow as a riffle, but is much slower-moving.

The pattern in which these four features occur depends on the soil-type, vegetation, slope, and amount of water flow. In most normal streams there is a sequence of shallow to deep water over a section of river. Scientists call this the riffle:pool ratio. In your watercourse project you may need to balance the riffle:pool ratio. Habitat biologists can help you determine whether this is necessary and if so, how to do it.

A healthy river will have a mixture of the four physical features. The manner in which they are mixed is unique for each stream, creating its individual character and appeal.
A healthy stream channel adjusts to change naturally without changing its basic shape and form
3.1.2 Typical Stream Channel Patterns

Gravity, friction, and depth of flow are the three main forces that create stream channels.

- Gravity causes water to move downstream and gives it speed, while friction between water and the streambed and banks creates a turbulence that slows down the flow.

 
- Friction increases in a stream when water flows over large materials such as rocks, boulders and debris. The size and amount of material determines the stream's "roughness"

- The speed of the water flow depends not only on slope and roughness of the streambed, but also "on the depth of flow". As the water level rises, the friction on the bottom and sides has less and less influence on the speed of the flow. Basically what happens is that water is now flowing over water and a stream is said to be "streaming". At this point, the internal strength of the water keeps it from pulling apart and causes it to slosh from side to side slowing the stream down. You can observe this effect by watching water run down a chute or a trough.

 


Natural streams are seldom straight. Streams curve around in a winding pattern or meander.

These meanders are formed by the way the water flows.

A combination of the forces described above create the physical features of a stream (riffles, pools, flats, and runs), and the shape of the channel.

The illustration below shows a typical pool/riffle sequence

If the stream is flowing over a mix of sand, gravel, or cobble (glacial till), it will form a nicely shaped channel.

The pools occur at intervals in the river, spaced 5 to 7 times the width of the river channel.

Over other kinds of stream bottoms, such as sand or boulders, the pattern is not as clear.

When the water is streaming, on the outside of a bend, the water is actually higher on the outside than inside of the turn. The water falls with gravity, under the main flow, to dig a pool. The heaviest material is dropped first, then gravel to build the head of a riffle, then silt and sand are deposited on the inside of the bend. Essentially at this point, the flow of the water is sorting the bottom material by size.

In shallow areas along the banks that go dry in low flow, vegetation, such as reeds, grasses, and willows, begin to grow. These plants also influence stream structure by slowing down the flow of high water running through them and by collecting debris, silt, and sand which build up the banks at these points, narrowing the stream. The roots stabilize the banks so as the water goes around a bend it digs down and under, creating deeper pools and undercut banks.

Logs falling in the stream often catch at the end of a riffle or the beginning of a pool because the flow of the water is slower here. These logs alter the stream bottom to giving the gravels of the riffle and run a firm base to pile up keeping it out of the pool. The small head difference across the log aids in deepening the pools. Logs and other large debris (called large organic debris or LOD) are essential in streams to create good pools, habitat diversity, and cover for fish.

Logs in the streams rot very slowly because they are always wet. Hardwoods withstand the impact of sand and gravels and are most commonly found as natural digger logs. Softwoods usually "pulp" quickly and are gone in a couple of years. Therefore, hardwood debris is very important in streams.

Three general terms are used to define the basic types of stream channels:.

Straight: Applies to sections or reaches of rivers that are relatively straight over a long distance. Such reaches are generally unstable. Even though the channel is straight, the water still bounces back and forth as it travels down the channel.

Braided: Applies to sections that have poorly defined, unstable and steep banks, and shallow watercourse with many channels around small islands. Too much sediment coming from tributaries or the crumbling, eroded banks often create the islands.

Meandering: Applies to sections with a single channel that has many bends, or "meanders", giving an "S" shaped pattern.

A stream will always attempt to maintain its meander shape and pattern. This curving pattern is formed by the water flood flows and is the pattern which is in balance with the substrate and banks to form a stable stream. Even when a stream channel appears straight, the line of its deepest point (thalweg) meanders back and forth across the channel in a predictable pattern.

You may wonder what the meandering of a stream has to do with adopting it. It is important to stress at this point that you will be working with a natural system that has its own natural patterns. In the Adopt-A-Stream program we are trying to work with a natural system and help it repair itself. If you don't understand the "natural flow" of the stream you are working on, you might end up working against nature, instead of with it.
When obstructions in a stream cause it to depart from the normal meander shape, the stream will always tend to try to get back to its natural meandering.
Here are some general principles that will help you understand a stream's meandering:

- The width of the stream and the length of the curves or meanders are closely related. If you imagine that a meander is a piece of string that you can straighten out, the length of the curve would be about 5-7 times larger than the width of the stream.

- When unstable stream banks become reinforced with rocks or plants the stream will tend to deepen and become more stable. At any given time streams are carrying all kinds of different materials such as particles of earth, plants, and debris. The way these materials are moved around and dropped off at various points affects a stream's shape and structure.

- When you look at a stream you probably think of the water as flowing in one direction. Actually, as water moves downstream it is also moving back and forth across the channel. This back and forth movement causes materials (like silt and sand) to be deposited or dropped at the inside of a curve, which creates a point bar. Floating debris builds up on the outside of the curve. For example, if a log or boulder were put on the inside bank of a curve, the structure would become buried. In your stream there may be need to help nature by strategically placing something that will improve fish habitat. This is covered in later sections of the manual and a habitat professional will help you decide on these locations.

- The physical structure of a stream and the quality of the water flowing in it creates a habitat or home to many forms of life. The illustration on the next page shows how a naturally shaped stream gives more useful living space. Making a stream straighter can dramatically alter the amount of useful space for fish and other wildlife.