The water in stream and river channels moves under the influence of gravity. In very slowly flowing streams, water moves in nearly straight-line paths parallel to the stream channel; this is called laminar flow (Figure 1A). However, streams typically exhibit turbulent flow (Figure 1B). Strong turbulent behavior occurs in whirlpools and eddies, as well as in roiling whitewater rapids. Even streams that appear smooth on the surface often exhibit turbulent flow near the bottom and sides of the channel, where flow resistance is greatest. Turbulence contributes to a stream’s ability to erode its channel because it acts to lift sediment from the streambed.

An important factor influencing stream turbulence is the water’s flow velocity. As the velocity of a stream increases, the flow becomes more turbulent. Flow velocities can vary significantly from place to place along a stream, as well as over time, in response to variations in the amount and intensity of precipitation. If you have ever waded into a stream, you may have noticed that the strength of the current increased as you moved into deeper parts of the channel. This is related to the fact that frictional resistance is greatest near the banks and bed of the stream channel.

Factors Affecting Flow Velocity

The ability of a stream to erode and transport material is directly related to its flow velocity. Even slight variations in flow rate can lead to significant changes in the load of sediment that water can transport. Several factors influence flow velocity and, therefore, control a stream’s potential to do “work.” These factors include (1) channel slope, or gradient, (2) channel size and cross-sectional shape, (3) channel roughness, and (4) the amount of water flowing in the channel.

Gradient

The slope of a stream channel expressed as the vertical drop of a stream over a specified distance is the gradient. Portions of the lower Mississippi River have very low gradients of 10 centimeters per kilometer (6 inches per mile) or less. By contrast, some mountain stream channels decrease in elevation at a rate of more than 40 meters per kilometer 211( feet per mile), a gradient 400 times steeper than the lower Mississippi. Gradient also varies along the length of a particular channel. When the gradient is steeper, more gravitational energy is available to drive channel flow.

Channel Shape, Size, and Roughness

A stream’s channel is a conduit that guides the flow of water, but the water encounters friction as it flows. The shape, size, and roughness of the channel affect the amount of friction. Larger channels have more efficient flow because a smaller proportion of water is in contact with the channel. A smooth channel promotes a more uniform flow, whereas an irregular channel filled with boulders creates enough turbulence to slow the stream significantly.

Discharge

Streams vary in size from small headwater creeks less than a meter wide to large rivers with widths of several kilometers. The size of a stream channel is largely determined by the amount of water supplied from the watershed. The measure most often used to compare the sizes of streams is discharge—the volume of water flowing past a certain point in a given unit of time. Discharge, usually measured in cubic meters per second or cubic feet per second, is determined by multiplying a stream’s cross-sectional area by its velocity.

The largest river in North America, the Mississippi, discharges an average of about 16,800 cubic meters (593,000 cubic feet) per second (Figure 2). Although this is a huge quantity of water, it is dwarfed by the mighty Amazon in South America, the world’s largest river. Fed by a vast rainy region that is nearly three-fourths the size of the conterminous United States, the Amazon discharges about 12 times more water than the Mississippi.

The discharge of a river system changes over time because of variations in the amount of precipitation received by the watershed. Studies show that when discharge increases, the width, depth, and flow velocity of the channel all increase predictably. As we saw earlier, when the size of the channel increases, proportionally less water is in contact with the bed and banks of the channel. Thus, friction, which acts to retard the flow, is reduced, resulting in an increase in the rate of flow.

Streams that flow year-round are called perennial streams and are typically fed by tributaries, groundwater, and precipitation. Streams that exhibit flow only during wet seasons are referred to as intermittent streams; during dry seasons, intermittent streams may not have flowing water. Rain-dependent streams, called ephemeral streams, carry water only occasionally, after a heavy rainstorm, and are most common in arid regions.

Changes from Upstream to Downstream

One useful way of studying a stream is to examine its longitudinal profile—a cross-sectional view of a stream from its source area (called the head or headwaters) to its mouth, the point downstream where the river empties into another water body—a river, a lake, or an ocean. As shown in Figure 3, the most obvious feature of a typical profile is a constantly decreasing gradient from the head to the mouth. Although many local irregularities may exist, the overall profile is a relatively smooth concave curve.

The change in slope observed on most stream profiles is usually accompanied by an increase in discharge and channel size, as well as a reduction in sediment particle size (Figure 4). Along most rivers in humid regions, discharge increases toward the mouth because as we move downstream, more and more tributaries contribute water to the main channel. In order to accommodate the growing volume of water, channel size typically increases downstream as well. Recall that flow velocities are higher in large channels than in small channels. Observations also show a general decline in sediment size downstream, making the channel smoother and more efficient (with less friction).

Although the channel slope decreases—the gradient becomes less steep—toward a stream’s mouth, the flow velocity generally increases. This fact contradicts our intuitive assumption that narrow headwater streams are swift, whereas a broad river flowing across subtle topography is slow. Actually, the increase in channel size and discharge and decrease in channel roughness that occur downstream compensate for the decrease in slope, making the stream more efficient (refer to Figure 4). Thus, the average flow velocity is typically lower in headwater streams than in wide rivers that appear to be placid.

• The flow of water in a stream may be laminar or turbulent. A stream’s flow velocity is influenced by the channel’s gradient; the size, shape, and roughness of the channel; and the stream’s discharge.

• A cross-sectional view of a stream from head to mouth is a longitudinal profile. Usually, the gradient and roughness of the stream channel decrease going downstream, whereas the size of the channel, stream discharge, and flow velocity increase in the downstream direction.

• discharge: The volume of water in a stream that passes a given point in a period of time.

• gradient: The slope of a stream; measured as the drop in vertical elevation over horizontal distance.

• laminar flow: The movement of water particles in straight-line paths that are parallel to the channel. The water particles move downstream, without mixing.

• longitudinal profile: A cross section of a stream channel along its descending course from the head to the mouth.

• turbulent flow: Movement of water in an erratic fashion, often characterized by swirling, whirlpool-like eddies. Most streamflow is of this type.