Chapter 3 - Streams in A Citizen's Guide to Understanding and Monitoring Lakes and Streams

One of the greatest things about streams is their endless variety. Some flow through riffle after riffle of deep forest, their banks dotted with moss-covered stones. Others flow through meadows, with deep pools and neatly undercut banks. Still others flow through cement culverts, along parking lots, and past the lawns and driveways of urbanized America. Of course, one stream may journey through all of these environments.

Streams are valuable recreation areas. For most adults the recreational value of streams may be limited to those with boating access or good fishing. But for children, any stream, even in the most industrialized portion of its length, represents the year-round playground of choice.

Streams are the workhorse of the local watershed. Carving through rock and streambanks, they create sediment that accumulates downstream as rich organic deposits. Nutrients, organic matter, and other pollutants the streams collect from the watershed are dispersed along their path. Everything deposited in the watershed – and every parcel of land is contained in a watershed – eventually will reach a stream and be carried away.

Because streams are constantly moving – cleaning, sweeping, and carrying things downstream – and are continually being replenished by rain and groundwater, they are self-cleaning. If we all do our part to remove pollutants from the watershed, streams will take care of themselves.

Stream water quality monitoring is one method available for determining the degree to which it has been affected by pollution or development. This chapter describes the importance of a stream’s physical characteristics and the importance and function of typical stream sampling parameters. Guidance also is presented on how to design a stream monitoring plan and how to analyze and interpret the data you have collected.

Organisms that convert coarse organic particles such as leaves and twigs into fine or dissolved organic matter.

Organisms that convert fine or dissolved organic matter into dissolved nutrients.

The Physical Character of Streams

No two streams are exactly alike – not even two segments of the same stream are exactly alike. Consider all the things that make a stream reach what it is: water velocity, depth, width, pools, riffles, vegetation, and the shape and nature of the shoreline. All of these physical characteristics influence water quality and the type and variety of habitat that is available to support aquatic life.

Stream Velocity Stream Characteristics - Copyright by Sandra Noel

Stream velocity is a measure of the water’s speed. A fast-moving stream is usually more turbulent than a slow-moving stream. The speed and extra turbulence give the water the force to scour the stream bottom and banks and pick up sediment and other material. The faster the stream is moving, the larger the materials it can pick up and carry with the current. In fact, algae and other organisms can’t live in a stream or stream section that is moving too fast because of this strong scouring force.

Stream velocity changes with season; in the Puget Sound Region this generally means faster during the winter and slower during the summer. Velocity also changes within stream segments; where the streambank widens or the channel deepens, the velocity decreases. Velocity also varies across the width of a stream. This is especially true when the stream is following a curve; the velocity is much greater on the outside of the curve than on the inside. The difference is often so great that while the force on the outside of the curve is strong enough to be cutting away at the bank, the force on the inside is so small that material is deposited along the bank.

Stream Depth

Stream depth determines the formation of pools, riffle, and glide areas. A pool forms in deeper segments while riffles form in shallow areas. A glide is the smooth, fast-moving area that often separates pools from riffles. Depth determines how much sunlight reaches the stream bottom, which in turn determines whether organisms that require light, such as algae, can grow there. The shallower a stream, the greater the proportion of water that is exposed to the air and sun. Exposure to the air where water can pick up more oxygen is good, but too much exposure to the sun can be harmful if water temperatures increase too much. Stream depth also varies with season, so that a segment that was a pool or glide during the winter may become a riffle during summer.

Stream Width

The narrower a stream, the greater the influence of streamside vegetation. A narrow stream may have a full canopy of trees or shrubs above it, as compared to a wide stream where the trees and other vegetation influence only the very edge. Bankside vegetation keeps temperatures cooler by creating shade, and provides places for fish and other organisms to hide.

Stream Discharge

Stream discharge refers to the total volume of water in the stream. It is a function of the cross-sectional area of the stream (width and depth), and the velocity. A wide, deep stream will have a greater discharge than a shallow, narrow stream, assuming their flow velocity is the same. Conversely, two streams of similar size may have quite different discharges if the flow velocity differs.

Shoreline Shape and Character

Some streams have sharp curves, others are straight. Some have high steep banks, others have gently sloping banks. Bankside characteristics range from exposed dirt, to rock, to thick vegetation. The shape and character of the shoreline affects how water moves past it, what vegetation grows there, and the type of habitat available. The change in the speed and force of water as it cuts around a curve further forms the shoreline and influences the pattern of riffles, pools, and glides. A straight or channelized stream is less stable and more prone to flooding than curving, meandering stream because of this distribution of energy.