Stream health is a term that is frequently used in the literature about streams. But health is seldom defined. The term makes an analogy to human health. Therefore, it does create a sense of stream condition that people can relate to. But it doesn’t really describe any particular state of the stream.
In the prologue of his very well researched and thought out book, ‘A View of the River’ (Harvard University Press, 2005), Luna Leopold makes it clear that the book is his view of the river, not an exhaustive treatise on the field of potomology (the study of rivers). This post is my very simplified view of stream health.
The World Health Organization describes health as ‘a state of complete physical, mental and social well-being’. That is a good place to start a view of stream health. It is likely streams have neither mental nor social well-being. I can’t really say that they don’t, but to be safe I will substitute physical, biological and chemical well-being. I do have some data on those characteristics.
With respect to physical well-being, streams have two basic functions: they transport water and they transport sediment. The amount of water transported by the stream, the stream’s discharge, is determined by the amount of precipitation in the stream’s watershed and possibly some contribution from groundwater. The amount of sediment is referred to as the stream’s load. Sediment is a product of erosion within the watershed as well as from the streams banks and bed. When a stream’s ability to transport sediment is equal to the amount of sediment provided by the watershed, then the stream is ‘in regime’. If the stream does not have capacity to transport the supply of sediment from the watershed, then deposition of sediment will occur. If the stream’s capacity is greater than the supply, then erosion occurs. When the stream is in regime, the stream flows along neither depositing nor eroding appreciable amounts of sediment. It will be a stable system.
A competent engineer can design a uniform channel that is in regime. That channel would flow along at a steady velocity and depth and sediment would gently bounce along the bottom. Little change would occur over time. The channel would also have few if any fish. Fish and the other members of the aquatic community need deep holes, shallow riffles, runs and glides. Different species occupy different environments. Our uniform channel needs some complexity to make the aquatic community healthy. A few engineers who are versed in fluvial geomorphology can design channels with rocks and other features put in just the right place to build in complexity and improve the aquatic habitat. Undescribed streams build in their own complexity. So, our healthy stream is in regime and also has complexity.
Biologically, my stream now needs a source of food for the aquatic community and a place for the citizens of that community to hide from predation. Where that food comes from depends on the location of the stream in the watershed. In the headwaters, streams are usually allochthonous (from without). The food washes in from the watershed or drops out of streamside vegetation in the form of bugs, worms and vegetative matter. Further downstream, where the stream widens and the canopy opens up such that sunshine hits the water, the food grows in the stream itself (autochthonous) in the form of algae. But nutrients (phosphorous, nitrogen, potassium) are still needed from the watershed to provide for growth. Again, the supply of nutrients needs to match the needs of the community. Too much nutrients and the stream gets fat, too little and it gets emaciated. A fat stream will likely get murky because of the algae content. And emaciated stream may be very clear but not have many fish. It’s a balancing act. However, like humans, streams will be in a better state of health on a lean diet. Then further downstream, the food mostly drifts down from above with the current (once again allochthonous).
The streamside vegetation is an important source of food for streams in the headwaters. Further downstream, the streamside zone also regulates the amount of nutrient reaching the water. So, streamside areas (riparian zones) are also important to well-being of the stream. Refugia from predation comes in the form of woody debris, rocks, cobbles, gravel and overhangs. This is frequently referred to by fishermen as structure.
The final characteristic of stream health is the chemical quality of the water. Water, even in a wilderness is a solution of many minerals and other compounds. In nature, the chemical characteristics of a stream are determined by the make up of the watershed that water flows through on the way to the stream and the chemical quality of the atmosphere that precipitation falls through. Most of the time, the stream will develop an ability to assimilate these minerals and compounds. In fact, a stream of distilled water would not be a healthy stream at all. The minerals and other compounds are necessary. When a chemical or other compound is added to a stream through point or nonpoint source discharges, the stream takes on the characteristics of those additions as well. If the stream cannot assimilate the addition, then it becomes polluted. Our healthy stream then must have a pollutant load less than the assimilative capacity.
In summary, my view of a healthy stream is one that is in regime (neither eroding nor depositing), has channel complexity, supports a diverse aquatic community with and adequate but not excessive nutrient supply, has a good cover of streamside vegetation, contains some structure and does not have an excessive load of chemical or organic pollutants. Such a stream should have a pleasant appearance, little nuisance vegetation, be a reliable source of water, and have good fishing.