Creeks and Pollutant Filtering

Small Streams Big on Cleanup

Creek advocates have argued for years that putting even the tiniest creek underground is a bad idea. Now there is new science to back up their voices. A National Science Foundation study just published in the journal Science reports that small streams do more than their fair share of work when it comes to filtering pollutants, often taking up and transforming more than 50 percent of the inorganic nitrogen entering the systems. Excess nitrogen - in runoff from fertilizers or byproducts of car exhaust - can be damaging when it reaches estuaries or other large bodies of water, where it can cause algal blooms and eutrophication. But before it gets there, small streams can remove a lot of it.

By placing tracers in small streams across the country, researchers were able to figure out how quickly pollutants were taken up - and compare their data with data on larger river systems. Small streams were found to be more effective at removing the pollutants than large streams. "Small streams get first crack at most non-point-source pollution because there are so many miles of small stream for each mile of large stream or river," explains Bruce Peterson, one of the researchers. "In addition, small streams remove nitrogen much more quickly because they are shallow. Most biological removal in small streams is by the stream bottom organisms. Where the water is shallow, as in small streams, these organisms have ready access to the nutrients in the water; where the water is deep, as in larger rivers, the nutrients must travel much farther before they are taken up."

Creek advocates aren't surprised by the findings. Says the Urban Creeks Council's Carole Schemmerling, "We've been pointing this out for a long time. About a year ago, a New York Times article reported that the Mississippi River is so heavily polluted that it can only be cleaned by restoring the smaller tributaries. Those smaller streams - if preserved and restored - can clean up the inorganic and organic pollutants that flow into the river. Saving these small streams is the only way to approach cleanup of a large body of water."

Peterson says land-use policies need to reflect the important role of small streams. "Remember that streams should function as part of an integrated landscape. If you put nitrogen fertilizer on a lawn or field, most of it should be retained in the crops or grass and soils, if you don't add too much or at the wrong time. Then the nutrients encounter a riparian zone of dense vegetation, and this zone also retains nutrients. Finally, the remainder enters the small streams, which in their natural or restored condition continue the removal process. If we neglect restoration and good management of the land and riparian zones, it is unlikely that the streams can do the whole job."

- courtesy of author, Lisa Viani. (A version of this article first appeared in the June 2001 issue of ESTUARY, published by the San Francisco Estuary Project.)