The Stream Doctor

October 6th, 2003

Email YOUR Questions directly to the Stream Doctor. This is your opportunity to get an experts professional opinion on anything stream related.

Q. From Claudia S.: I would like to ask you where are most mayfly nymphs found in freshwater rivers, in the middle or edges? Also is the middle of the river faster than the edge? And if so will it have more dissolved oxygen? And will this enable a greater population of mayfly nymphs? Also is the depth of the river greatest at the middle or edge? And does the depth affect the velocity and number of mayfly nymphs?

Your help is much appreciated. Many thanks.

A. Claudia, you've asked several questions having to do with two general themes: (1) river channel shape and form (geomorphology) and (2) distribution of mayfly nymphs. I'll address your questions under these themes and hope to answer your questions.

River channel geomorphology: A natural, meandering river exhibits several characteristics pertaining to depth, current velocity, and channel form. A line connecting the deepest points along a river channel is called the thalweg. Now this line can shift back and forth as the channel meanders; where the channel swings against a cut bank, the thalweg is closer to the bank than to the middle of the channel. In a straight section of the channel, the thalweg is usually near the center of the channel. Thus, the deepest part of the channel shifts back and forth as the river meanders; it is deepest near the edge on the outside of a curve, shallowest on the inside of the curve, and deepest, usually, in the middle on a straight reach. Current velocity is generally greatest along the thalweg because there is less drag (friction) of the entire water column against the bottom where it is deepest. In most average size streams and rivers, dissolved oxygen will not vary much from side to side; the mixing of the water as it tumbles over rocks ensures adequate exchange of oxygen with the air and within the water column.

Mayfly nymph distribution: This is a little harder to answer directly because the answers to your questions can vary depending on the size of the river you are talking about and the number of mayfly species indigenous to the area. There are many types of rivers and over 700 different species of mayflies, so it is difficult to get very specific. Mayfly nymph distribution across the width of a stream or river can vary with the size of the river. A small headwater stream or mid-size river can have a fairly uniform distribution of rocks, pebbles, etc. from edge to edge. Given these uniform conditions, it is unlikely that the distribution of mayfly nymphs will be much different, in either numbers or species or number of individuals per unit area, across the bottom. However, as rivers get bigger, several conditions may change across the width of the river. Water current can be greater in the middle, and the distribution of bottom materials can change, thus presenting different habitat conditions in different parts of the river. Thus, you might find a situation where the faster current in the middle of a river keeps the substrate cleared of sand and silt. Here, then, you will find larger stones and rocks that are suitable habitat for certain species. In the shallow areas along the edges of the stream, sand and silt may accumulate, thus providing habitat for those mayfly species adapted to live in these conditions. Will there be more mayfly nymphs in the middle or edges? This depends on the species present, the relative amount of habitat available (more rocks or more mud), and the available food in each area. Different mayfly nymphs require different kinds of foods, e.g., algae on rocks for the grazers, suspended particulate matter for the filterers, and deposited particulate matter for the collectors. The combination of flow, light conditions, and input of particulate organic matter dictates which types of food are present and in what amounts. Mayfly nymphs are sensitive to amounts of dissolved oxygen present in the water, so this, too, will influence the numbers that you find in different habitats.

I've only generalized for a couple of situations above; to go into more detail would take an undue amount of space. If you'd like to pursuer this further with more specific questions, please feel free to contact me.
~ C. E. (Bert) Cushing, aka Streamdoctor
105 W. Cherokee Dr.
Estes Park, CO 80517
Phone: 970-577-1584

The 'Stream Doctor' is a retired professional stream ecologist and author, now living in the West and spending way too much time fly-fishing. You are invited to submit questions relating to anything stream related directly to him for use in this Q & A Feature at

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