Water Currents: A New Check-Up on the Health of U.S. Rivers
Posted 24 July 2013 - 09:58 AM
But the vast majority of the nation’s rivers and streams still do not measure up as healthy.
According to a new assessment by the U.S. Geological Survey (USGS), less than one in five streams near agricultural or urban areas gets a good bill of health.
Dams and diversions that alter streamflows, along with pollution from chemical fertilizers are among the key factors degrading rivers.
A More Complete Look at River Health
To assess the health of rivers, the team of USGS scientists examined communities of algae, macroinvertebrates (e.g., aquatic insects, snails and mussels) and fish in relation to human-caused changes in streamflow and water quality. The status of these biological communities adds crucial information because it shows the degree to which freshwater life is able to adapt, or not, to changes in a river’s physical and chemical conditions over time.
In 83 percent of the streams assessed by the USGS team, at least one of these three communities of organisms was altered.
Changes in the timing and magnitude of natural streamflows – often caused by dams, diversions, levees and groundwater withdrawals – can severely impact freshwater life.
Every river has a natural pattern of flow— highs and lows, floods and droughts – to which fish and other river creatures have become adapted over time. Aquatic communities depend on the habitats these variable flows create and the life-cycle cues they provide. [See my 2010 post, Why Rivers Need to Flow – High and Low – Again]
The USGS team, led by ecologist Daren Carlisle, found that reductions in annual high flows caused the incidence of altered fish communities to increase from 12 percent to 40 percent. A similar level of impact was found for macroinvertebrates.
In summer, lower-than-normal low flows not only alter physical habitats, they can cause water temperatures to rise to levels harmful to trout and other cold-water fish.
For example, in Colorado’s Cache la Poudre River, which flows from the Rockies east across the Colorado plains, large diversions for irrigation and water supplies have depleted low flows in some stretches to less than 10 percent of normal. Instead of cutthroat trout and other species that need fast-flowing water, the depleted Cache la Poudre harbors green sunfish, fathead minnow and other fish adapted to intermittent or slow-flowing streams.
The USGS team also found that elevated stream levels of nitrogen and phosphorus – often from fertilizer runoff and wastewater discharges – caused the incidence of altered algal communities to rise from 21 percent to 39 percent. These high “nutrient” levels can cause so-called algal blooms that deplete oxygen and, especially during warm summer months, can lead to fish kills.
Taken together, these impacts on river health place at risk a whole suite of benefits human communities derive from healthy river systems – from water purification and waste decomposition to wildlife habitat and recreation. Sport fishing alone contributes an estimated $125 billion a year to the U. S. economy.
Solutions for Healthier Rivers
Boosting the health of the nation’s rivers requires changes in how we use and manage both land and water. Protecting streams from farm runoff – by planting vegetative buffers, for example – can reduce nitrogen and phosphorus pollution.
Operating dams and reservoirs so as to some degree mimic natural patterns of river flow can help preserve the habitats and food webs so crucial to aquatic life, often while improving economic returns from the river at the same time.
The Sustainable Rivers Project, a partnership of The Nature Conservancy and the U.S. Army Corps of Engineers, has been piloting such an effort in eight river basins across the country, including the Connecticut, Green, Roanoke and Willamette. The aim is to transfer the knowledge gained from these demonstrations to more of the 600 dams managed by the Corps of Engineers – dams that collectively impact some 50,000 miles of U. S. rivers.
In addition, the removal of obsolete or unsafe dams is restoring more-natural flows to rivers and opening up vital habitat to fish and other aquatic life. This week, the dismantling of the Veazie Dam on Maine’s Penobscot River got under way – part of a large restoration effort on the Penobscot that will open access to some 1,000 miles of habitat for Atlantic salmon.
In urban areas, such as Philadelphia and Seattle, the push for porous pavement, bioswales, rooftop gardens and other “green infrastructure” is helping improve flow conditions and water quality in nearby rivers and streams.
While much work remains to be done, the glass-half-full message of the USGS report is that nearly one in five streams near agricultural and urban areas have healthy biological communities.
When we put our minds to it, thriving cities and productive farms can exist side-by-side with healthy rivers.
Sandra Postel is director of the Global Water Policy Project, Freshwater Fellow of the National Geographic Society, and author of several books and numerous articles on global water issues. She is co-creator of Change the Course, the national freshwater conservation and restoration campaign being piloted in the Colorado River Basin.
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