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Rivers offer areas for swimming, boating, fishing, or waste disposal, and sources of water for generating energy, drinking, snowmaking, irrigating crops or fighting fires. But they are more than their sum of human uses. They are "home" to an interconnected web of creatures, many of which we never see and rarely think about. Yet, many of the uses we make of rivers have tremendous impacts on natural communities, making the health of rivers our responsibility.
Once we begin to consider the river as a web of living organisms bound together by water, we should begin to wonder how river users affect how that biological community functions. We can think of river communities as having physical, chemical, and biological attributes:
and foundation for a river community is flowing water and its relationship to the land area that drains into the river - its watershed. It's water rushing through a gorge, or flowing lazily through a farm meadow. It is a physical process, as runoff flowing over land and groundwater flowing underground replenish the river with water, sediment, and nutrients. In turn, the river influences the land by cutting its channel through rock and soil, and carrying the eroded material downstream. Shallow rapids where water flows over submerged rocks or riffles, calm gentle glides, deep slow pools, and stagnant waters along the river margin or backwaters, provide a variety of habitats for fish, waterfowl, and aquatic invertebrates.
are the basic building blocks for river life. These are the water's oxygen content (dissolved oxygen or DO), acidity (pH), ability to neutralize acid (alkalinity), nutrients, metals, and other constituents. In the absence of human influence, the water chemistry is determined by the soils and rocks in the watershed, the chemistry of the precipitation, and interaction with plants and animals on land and in the water. It profoundly affects, and is affected by, aquatic organisms.
| of river communities are wonderfully varied - from single-celled plants and animals, aquatic insects, and other small residents, up through large fishes and waterfowl. Flowing water is the thread that binds this living community together within itself and with the surrounding land. How does this web of life work? |  |
A leaf falls into a small stream high in the mountains. It is quickly attacked by bacterial, fungal, and macroinvertebrate decomposers. Some of the nutrients in the leaf are dissolved into the water, and flow downstream until taken up by aquatic plants. Aquatic insect shredders (such as caddisfly larvae and snails) feed on the leaf and its attached "frosting" of decomposers.
Meanwhile, grazers (such as mayfly nymphs), feed directly off living aquatic plants. Grazers and shredders reduce plant tissue to smaller particles, some of which is used by the insect to grow.
Excreted or unused food is washed downstream. This "detritus" provides food for the collectors such as black fly larvae and worms, which are waiting downstream to catch a meal. The insects themselves provide food for other predatory insects and fish. As the river flows downstream, some organic material is stored (as insect or animal tissue), some is cycled (changed to different forms), and some is released. Downstream, aquatic communities take advantage of inefficiencies upstream.
As you can tell from the
above cycle, river communities are not homogeneous, nor are they static. A river
community changes dramatically from its headwaters to its mouth, from season
to season, and from year to year. However, healthy rivers are remarkably stable
communities due to the diversity of organisms living there. A diverse river
community indicates that a menu of food choices is available. Reducing these
choices by physical or chemical alteration decreases community diversity. Since
many aquatic organisms are opportunistic, they can adapt to changes in the food
supply. Organisms that are most effective at using the remaining food sources
will dominate, others may disappear.
Serious disruptions may eliminate large segments of the community. This is what is meant by loss of biodiversity. Human Impacts on River Communities People can affect rivers directly by dumping things in them or changing their channels or indirectly by changing the land through which the rivers flow.
To see how, let's assault a hypothetical river. First, we'll cut down the trees along the banks and pave the watershed. Then we'll add sediment and nutrients from urban areas, crop land, lawns and golf courses. We'll dam the river, dump sewage into it, pump water out of it, and, for good measure, excavate the bed for sand and gravel. What have we done to this river community? A whole chain of events is set in motion throughout the aquatic ecosystem.
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from the banks means less food from leaf fall, and warmer water temperatures as the sun strikes and heats more of the water surface. Since most biochemical processes speed up as the temperature increases, warmer water can push the system into high gear. Decomposers work and use oxygen at a faster rate. To compound the problem, warm water holds less oxygen than cold water. Some species can't survive in warm water, either due to lower oxygen levels, or because they are sensitive to heat. Also, loss of large woody debris such as fallen logs and root wads removes hiding and resting areas for fish, known as "cover." |
parking lot, and roof surfaces
speed up the over-land flow of water, which means more water gets to the river
faster. Higher flood flows during spring and fall, more channel scouring, and
more erosion (since there's more energy and soil particle-dislodging potential)
are the result. Less water is able to infiltrate the soil to replenish groundwater,
so lower summer flows occur. Asphalt surfaces also heat the water entering the
river. Pollutants such as oils, trash and even dirt are washed right into the
river instead of being filtered through the soil.
Now let's add some sediment from eroding crop land, construction sites, logging areas, and bank erosion. Sediment buries the homes and clogs the gills of many aquatic organisms, and catches and holds heat from the sun, causing warmer water temperatures. Often, nutrients and other pollutants, including heavy metals and toxics, attach to sediment, adding to the damage it causes when washed into streams. Habitat alteration from sedimentation is one of the leading causes of fishery decline.
Adding nutrients to a river encourages aquatic plant and algae growth. These plants may create wild swings in oxygen levels, as they release oxygen during the day and along with aquatic animals and decomposers consume it at night. When the plants and algae die and decompose, more oxygen is used. Again, aquatic organisms may suffocate, especially in the dark hours of early morning.
changes the physical foundation of the river when it replaces rapids and cascades with a reservoir. Above the dam, water velocity is slowed, causing soil particles and organic material to settle to the bottom and cover the river bed with mud. Oxygen levels are reduced as the organic material, which previously flowed downstream, decomposes. At the same time, oxygen is not replenished as quickly in the reservoir as it was in the rapids.
Downstream of the dam, the flow and oxygen levels may fluctuate dramatically, if water is daily stored and released. The food supply from upstream is reduced. Temperature changes may cause a trout fishery to be succeeded by a warm-water bass population.
means decomposing organic material, which consumes oxygen and adds nutrients to the water. The decomposers go to work. As they work, they breathe in oxygen, breathe out carbon dioxide and create an oxygen demand. If we put too much organic material in, the river can't replace oxygen fast enough and the demand exceeds the supply.
Fish like salmon and trout need high oxygen levels in the stream to survive. So do the egg and juvenile life stages of other fish species. Aquatic organisms that need the most oxygen will suffocate first, reducing the food choices for surviving species. If the oxygen drain continues, more species will perish, reducing the food choices further. In Massachusetts, late summer is a stressful time for fish, as water levels drop, water temperatures rise, and dissolved oxygen levels decrease. Added organic or nutrient loads can cause fish kills. Sewage also brings in bacteria and pathogens, making the water unsafe to drink or swim.
in late summer can be exacerbated by withdrawals of water from the river directly or through groundwater wells. Shallower waters and lower flows mean less living space is available to fish and other aquatic organisms, increasing competition for food and predation. Lower oxygen levels and increased temperatures stress fish, making them more susceptible to bacterial infections.
or bucket loader pulling gravel out of the stream changes the physical habitat available for some insect and fish species in addition to causing sedimentation by stirring up the streambed. Not only do these individual actions cause impacts, there is a cumulative impact that may go far beyond the sum total of the individual impacts. Most any river in Massachusetts has been subjected to this treatment at some time it its relationship with humans. This not-so-hypothetical river can no longer support the same aquatic community it once did.
Monitoring will help you identify the changes that may be occurring in your river or stream as well as lead you to ways to protect and restore it.
