Blurring the Boundary

Leaves of autumn. Boundaries may be lines, walls, roads, or shorelines. The water/land boundary of rivers is a place of exchanging energy. Leaves, particularly in autumn, fall into the river, providing food for invertebrates, fungi, and bacteria. The leaves are

Leaves broken down into coarse particles, first by shredders such as certain caddisfly or mayfly larvae, then the food (sometimes thought of as energy) becomes smaller and smaller pieces of fine particles gathered in by the likes of net spinning caddisflies and black fly larvae. Eventually the leaves become suspended or settled organic food stuff fed on by midges, for example and then broken down by fungi and bacteria into forms of nitrogen and phosphorus.

Food web 3
Roots representing  parts of a web.

The webs of life. These essential nutrients feed small algae that grow on the rocks or sand or mud or just float in the river. The algae produce oxygen for the invertebrates and fish to breath and provide another source of food. Fish feed on the invertebrates or algae or other fish. This, a smorgasbord of food, energy, and nutrients spirals in the river, in place and then heads downstream. A lovely world of shredding and grazing and gathering and breathing, hidden under the surface of the water, unknown to most people.

What lies beneath
What lies beneath?

A river runs through it, Those who fly fish are aware of this world. They make use of their knowledge of the different forms of aquatic insects to tie shapes on their fishing hooks. Then in an act of performance art they cast into the river, mimicking the movements of larvae or pupae or adult insects in the hope of luring a fish to the hook. Perhaps one of the best descriptions of this kinetic poetry is by Norman Maclean in A River Runs Through It. The knowledge of aquatic ecology possessed by some who fly fish rivals the that of the most knowledgeable of aquatic ecologists. Fly fishing works because the energy exchange across the water/land boundary or ecotone goes both ways, with energy emerging from the water and into the land or terrestrial environment, often in the form of adult insects.

To mate and die, The energy literally emerges from the water in the form of adult aquatic insects leaving their natal or birth environments to mate in air only for the females to deposit or oviposit her eggs back in the water. If you live near a river you may be familiar with large emergences of heptageniid mayflies, adults filling the air, swarming, mating, and falling dead back to the land and surface of the water. Fish have a feast, but so do many land animals ranging from birds to shoreline spiders and mammals. All that energy gathered in by the insect larvae from leaves and algae and from eating the insects that feed on leaves and algae is now shared with the land, crossing the land/water ecotone. Large swarms of adult midges may emerge in synchrony, so dense and large they appear as a cloud on the water or along the shore. The males swarm, waiting for females to fly through the swarm, to mate, to die. Dragonfly and damselfly adults fly on the hunt and are hunted themselves. Bats swoop down at dusk to feed on the aerial provender. Eventually, all the insects and birds and bats and mammals pass along the energy to the land as they die or defecate. Fungi and bacterial feast and pass along essential nutrients, nitrogen and phosphorus, to the land along the river, the riparian zone.

West Fork Bitterroot_Sept_2018d
West Fork of the Bitterroot River. This narrow river has a large riparian zone relative to its width with abundant energy inflow from the riparian vegetation.


Rivers swell and shrink. Rivers swell with the pulse, with the waters of the spring floods, then as the weather turns warm and dry, rivers diminish, flowing within their banks once more. In nature, the spring swell flows into nearby or riparian wetlands in places where the floodplain spreads out. These wetlands blur the boundary between water and land. The water flows over grasses, inundates the roots of riparian trees such as cottonwoods and burr oaks and willows, to reside in the wetlands for days, weeks, months, or years. Now the give and take of food and energy is more intimate, more immediate. The wetland water is in, on, and among the leaves, grasses, soils that provide the food and nutrients. The slow moving or standing water is an excellent habitat for algae, feeding more invertebrates and fish than the nearby river, exchanging this food and energy with the river and receiving water in return. The river water often flows over land to enter the wetlands but may also exchange under ground in an interstitial connective network of water flowing between the subterranean particles of rock, gravel, sand, mud, or clay.

Thank your local riparian wetland. With more food and energy, the wetlands produce more energy and food in return, sending the energy into the surrounding riparian zone. Although the poor hobbits suffer by walking through the midgewater marshes, those midges (more like biting midges from Tolkien’s description) feed the birds and bats and spiders, sustaining a complex ecosystem. The natural ebb and flow of the spring flood pulse brings more nutrients downstream to reside in riparian wetlands. The wetlands provide a service, retaining the water and energy locally for a while before slowly releasing the water and energy back downstream or to the nearby land. The wetlands also provide habitat for wildlife such as deer, wild turkeys, and migratory water birds. So, if you get a chance, go visit your nearest riparian wetland. Indulge in the beauty of these natural energy producers and please take a moment to reflect on the services they provide to all.

Riparian wetland at Lee Metcalf Wildlife Refuge

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