The source of a first-order or “headwater” stream (top), and the first-order stream itself (bottom).
Water gets from its source to cities and the ocean via the “stream-order continuum.” The stream-order continuum operates on a simple premise: Streams are Nature’s arterial system of the land. As such, they form a continuum or spectrum of physical environments, with associated aquatic and terrestrial plant and animal communities, as a longitudinally connected part of the ecosystem in which downstream processes are linked to upstream processes.
The idea of the stream-order continuum begins with the smallest stream and ends at the ocean. The concept centers on the resources of available food for the animals inhabiting the continuum, ranging from invertebrates to fish, birds, and mammals—including people.
As organic material floats downhill from its source to the sea, it becomes smaller and smaller in size, while the volume of water carrying it becomes larger. Thus, small streams feed larger streams and larger streams feed rivers with partially processed organic matter, such as wood, the amount of which becomes progressively smaller the farther down the continuum of the river system it goes.
This is how the system works: A first-order stream is the smallest undivided waterway or headwaters. As such, a first-order stream is the only aquatic entity with ecological integrity because it is not influenced by the condition of any other stream. Where two first-order streams join, they enlarge as a second-order stream. Where two second-order streams come together, they enlarge as a third-order stream, and so on.
The concept of stream order is based on the size of the stream—the cumulative volume of water, not just on what stream of what order joins with another stream of a given order. To illustrate, a first-order stream can join either with another first-order stream to form a second-order stream or it can enter directly into a second-, third-, fourth-, fifth-, or even larger-order stream. The same is true of a second-order stream, a third-order stream, and so on.
A third-order stream in the mountains (top), and a fifth-order stream—river—in the valley (bottom).
In addition, stream-order influences the role of streamside vegetation in controlling water temperature, stabilizing banks, and producing food. For example, streamside vegetation in forests is the primary source of large, organic debris, such as tree stems at least 8 inches (20 centimeters) in diameter with their rootwads attached, or tree branches greater than 8 inches (20 centimeters) in diameter. Erosion also contributes myriad organic materials to the stream.
Wood in streams increases the diversity of habitats by forming dams and their attendant pools and by protecting backwater areas that are important winter habitat for fish. In addition to the wood itself, habitat diversity in the streams and rivers of the western United States has been historically maintained by regular flooding, droughts, and every imaginable condition in between these extremes; the same is true for virtually all countries.The variability of the conditions experienced by the streams and rivers continually shift the wood around and alter its function in a way that augments ecological diversity in space and time, thereby causing indigenous organism to evolved in ways that allow them to cope with the extremes of survival. Three examples are: cottonwood trees, a caddisfly, and a giant waterbug.
Cottonwood trees in the Southwestern United States, which once grew in profusion along the banks of streams and rivers, where they provided shade, woody debris, and nutrients to the aquatic-terrestrial interface, have all but disappeared to the detriment of the ecosystems they served. Cottonwoods require the bare, scoured banks that result from floods in order for their seeds to germinate and grow, despite the fact that some mortality of the trees themselves is experience as a consequence of the flooding. Today, cottonwood trees are dying out in many areas—and the free, ecological services they performed with them—because of flood-controlling dams.
Wood in streams increases the diversity of habitats by forming dams and their attendant pools and by protecting backwater areas that are important winter habitat for fish.
There is a caddisfly that inhabits a stream system in the mountains of Arizona, where it is subjected to the extremely violent force of flash floods, which occasionally scour out the stream channels. The caddisfly, in turn, has evolved through the generations to metamorphose from the immature, aquatic state into their winged, adult phase during a period that is almost perfectly timed to miss the most common season of flooding, which keeps enough of the population out of harms way to perpetuate the species.
Finally, a giant waterbug, which lives in some desert streams, has adapted over the last 150 million years to “read” the weather and make a mass exodus from a stream that is about to experience a flash flood. During the exodus, the waterbugs literally climb the canyon walls to escape the dangerous waters, but return to the stream within a day.1
When rivers are “harnessed” and “tamed” with dams, the organisms that have evolved to cope with Nature’s disturbance regimes are likely to die out and be replaced by a range of different organisms. The shift in habitat and the attendant aquatic organisms that result from the construction of dams can dramatically alter how the ecosystem functions in a way that is detrimental to the food web within the entire drainage basin affected by the dams, such as preventing driftwood from completing its journey from the forest to the sea. Conversely, when streams and rivers are unrestrained, the driftwood they carry provides nutrients, a variety of habitats for biological activity, and both dissipates the energy of the water and traps its sediments.
Processing the organic debris entering the aquatic system includes digestion by bacteria, fungi, and insects that are aquatic in their immature stages (such as midges, stoneflies, mayflies, and craneflies) as well as physical abrasion against such things as the stream bottom and its boulders. In all cases, debris is continually broken into smaller pieces that make the particles increasingly susceptible to microbial consumption.
The amounts of different kinds of organic matter processed in a reach of stream (the stretch of water visible between two bends in a channel, be it a stream or river) depends on the quality and the quantity of nutrients in the material and on the stream’s capacity to hold fine particles long enough for their processing to be completed. The debris may be fully utilized by the biotic community within a reach of stream, or it may be exported downstream.
Aquatic algae is one source of organic matter in a stream that both acquires a true investment of energy from the sun through photosynthesis, thereby creating food for living organism, and recycling a combined investment and reinvestment of nutrient capital (chemical elements and compounds) when it dies.
Debris moves fastest through the system during high water and is not thoroughly processed at any one spot. The same is true in streams that do not have a sufficient number of instream obstacles to slow the water and act as areas of deposition, sieving the incompletely processed organic material out of the current so its organic breakdown can be completed. Moreover, as a stream gets larger, its source of food energy is derived increasingly from aquatic algae and less from organic material of terrestrial origin. The greatest influence of terrestrial vegetation is in first-order streams, but the most diversity of incoming organic matter and the greatest diversity of habitats are found in third- to fifth-order streams and large rivers with floodplains.
Small, first-order, headwater streams largely determine the type and quality of the downstream habitat. First- and second-order streams are influenced by the configuration of surrounding landforms and by the live and dead vegetation along their channels. This riparian vegetation interacts in many ways with the stream.
Rocks of various sizes and shapes, in addition to wood, play a critical role as instream habitat—note the strings of green algae swirling around its base.
The canopy of vegetation, when undisturbed, shades the streamside. The physical energy of the flowing water is dissipated by wood in stream channels, slowing erosion and fostering the deposition of inorganic and organic debris. These small streams arise in tiny drainages with a limited capacity to store water, so their flow may be scanty or intermittent during late summer and autumn, but during periods of high flows in winter and spring, they can move prodigious amounts of sediment and organic material.2
However, over much of the globe today, high-elevation, forested water catchments that once protected the snowpack from the heat of the sun have steadily given way to commercial logging—legal and illegal. As logging roads progressively fragment the once contiguous forests and clear-cut after clear-cut merge into gigantic, naked mountain slopes, the snow melts earlier and faster, and in so melting saturates the soil in a short time. In many locations, the water-holding capacity of the soil is often reached in late May and early June, greatly exceeding gravity’s ability to pull the water through the soil into the valley bottoms and thus allow the soil to absorb all the water. The inability of the soil to absorb the great pulse of water causes most of it to flow over the surface of the ground, where it rushes down streams and rivers, speedily fills reservoirs to overflowing, and so is lost to the human communities when they needed it most, late in the year.
To help you visualize what I am talking about, consider a large, porous, rotting log, with both ends cleanly cut off, lying across the contours of a steep slope (up and down the slope) under the canopy of an ancient forest. If the snow is deep enough, the melting water infiltrates the log at its upper and is gradually pulled downward through its interior by gravity until it drips out the bottom of the cut face at the log’s lower end.
There is, however, a caveat to this phenomenon. If the snow is deep enough to cover the upper end of the log, it can absorb the same amount of water that drips out the bottom just as long as the supply lasts. But, as soon as the snow is gone, the available supply of water is cut off, and that remaining in the log will eventually drip out the lower end without being replenished. Therefore, the longer the snow last at the upper end of the log, the longer the log can act as a conduit for the water infiltrating its upper end, passing though its length, to drip out its lower end. Conversely, the faster the snow disappears from the log’s upper end, the faster the supply of water from melting snow is cut off, the quicker the log progressively dries out, even as water continues to drip out the lower end. That, too, will shortly cease because, without the water stored in the snowpack above ground to cover the log’s upper end, there is no replenishment for the limited supply of water pulled through the log by gravity.
So it is, when considering the supply of water for communities, that humility, wisdom, and long-term economics dictate that some forested water catchments, particularly at high elevations, should not be cut even once for the perceived, immediate, short-term dollar value of the wood fiber. To protect such areas for the storage of water in the form of snowpack will require a drastic shift in thinking because, at present, the only economic value seen in high-elevation forests is the immediate extraction of wood fiber. Nevertheless, the expanding network of roads and clear-cuts in high-elevation forests, which capture and store water, affects all human communities, from the smallest rural village to the largest city.
In addition to the beneficial aspects of the stream-order continuum, however, there is a sinister side this story as well—a tragically human side. Ditches along forest roads (and elsewhere) form a continuum or spectrum of physical environments (the same as streams), a longitudinally connected part of the ecosystem in which downstream processes are linked to and influenced by upstream processes. The ditch continuum begins with the smallest ditch and ends at the ocean. So it is that little ditches feed bigger ditches, and bigger ditches eventually feed streams and rivers that ultimately feed the ocean. Further, that as organic material (food energy) floats downhill from its source to the sea, it gets smaller—more dilute—as the volume of water carrying it gets larger.
Here the question is: What happens to the continuum concept when a ditch is polluted? To pollute a ditch means to contaminate it by dumping human garbage into it or by consciously or unconsciously discharging noxious substances into it, such as oil or hydraulic fluid from vehicles and logging equipment, both of which in one way or another disrupt biological processes, often by corrupting the integrity of their chemical interactions.
A rare roadside ditch that is still clean enough for frogs and salamanders to live in.
While Nature’s organic matter (food energy) from the forest is continually diluted the further down the stream continuum it goes, pollution (especially chemical pollution) is continually concentrated the further down the ditch continuum it goes because it gathers its potency from the discharge of every contaminated ditch that adds its waters to the passing flow. Hence, with every ditch that is polluted, the purity of the stream and river accepting its fouled discharge is to that extent compromised, and the amount of pollution that ends up being dumping into the estuaries and oceans of the world through the stream/ditch continuum is staggering.3
I say this for two reasons. First, I have seen ditches in North America, Europe, Asia, and Africa discharging their foul contents directly into streams, rivers, estuaries, and oceans. Second, in 1969, I found a population of montane voles (meadow mice to most people) living along a ditch that drained an agricultural field. The voles, whose fur was an abnormally deep yellow when I caught them, lost the yellow with their first molt in the laboratory when fed normal lab chow; whereas those along the ditch retain their yellow pelage.4
Even with evidence in hand, I could find no one in the Department of Agricultural Chemistry at the local university to acknowledge this color deviant, let alone examine it in a effort to find the cause—undoubtedly some agricultural chemical compound, which, if a fertilizer or herbicide, could just as easily be a chemical compound used in exploitive forestry. Nevertheless, they all turned their backs, even when I presented them with the evidence, the live, yellow voles.
So I learned that chemical pollution in ditches is not visible to the eye of human consciousness in the flowing of their waters, but it may become visible in the sickening of the environment. And in 1984, as part of a committee called to Washington, D.C., to help the United States Congress frame the ecological components of the 1985 Farm Bill, I learned in far greater depth of the incredible amount of non-point-source chemical pollution of our nations surface waters (ditches, streams, rivers, and lakes) and groundwaters (lakes and aquifers) from today’s chemical-intensive agricultural, including exploitive forestry.
How, I wonder, can we learn to care for rivers and oceans if we continually defile the ditches that feed them? The answer is that we cannot!
We must learn to care first and foremost for the humble things in our environment, such as a roadside ditch, before we can learn how to care for the mighty things in our environment, such as a river. Defile the ditch, and we defile the stream, river, estuary, and ocean; protect the ditch and we protect the stream, river, estuary, and ocean in like measure.
The above discussion refers to water that flows for a time through the soil and completes its journey to the ocean above ground. There is, however, a belowground analog in subterranean seeps, trickles, and rivulets, which coalesce into streams and rivers that flow from the mountains to the ocean entirely below ground. On reaching their destination, this sub-ocean flow eventually emerges at submarine fresh-water springs or seepage zones at varying depths on the ocean floor where the fresh water influences the dynamics of the marine ecosystem. Around 480 cubic miles (2,000 cubic kilometers) of fresh water enter the world’s oceans each year as submarine groundwater, although some coastlines providing considerably more than others.
The pressure of ocean water does not control the groundwater discharge. In fact, a submarine spring can flow equally well whether in shallow coastal waters or at the bottom of a deep ocean trench. There are, however, three specific conditions required for a significant submarine flow of offshore groundwater. First, there must be sufficient precipitation in the region to sustain the supply of groundwater. Second, the subsurface geological materials (aquifers) through which the water flows must be permeable enough to allow the easy seaward movement of the water. Third, the source of the groundwater must be sufficiently high in elevation to provide a pressure gradient strong enough to push the water along in aquifers extending outward from the land beneath the sea.5 In addition, the volume of submarine-groundwater discharge represents an important vehicle for the delivery of nutrients, carbon, and metals to the ocean.6
The hidden waterways, like their aboveground counterparts, are conduits that continually deliver pollutants from the land to the oceans of the world. Here, petrochemicals from agricultural fields and tree farms, from urban settings and industrial complexes leach into the deeper terrain of the soil, where they are entrained in stream and river to enter and defile the mother of all waters.
WATER–A CAPTIVE OF GRAVITY
Text and Photos © by Chris Maser, 2009. All rights reserved.