Deluged

Shaver's strategy - which recognizes the value of trees in helping manage runoff - is one many states will emulate, predicts Don Woodward, hydrologic engineer with the Natural Resource Conservation Service. NRCS, long a proponent of using green space to manage runoff, is helping states and communities learn to use trees and other natural nonstructural techniques as an alternative to retention ponds and traditional stormwater containment facilities.

It's not hard to see why Shaver's way of thinking could easily go national. The philosophy is logical and based on sound, solid science - trees help slow and cleanse runoff, more natural, undamaged soils quickly soak up rainwater and help prevent flooding. Stormwater runoff costs billions to manage each year and causes billions more in flood damage. As city leaders struggle to keep up with burgeoning urban populations, why not expand stormwater management strategies to include natural processes?

Strategically planting trees within the city limits and in surrounding suburban and rural areas upstream reduces the flow of stormwater and even improves the quality of the water washing into lakes and bays. This is particularly important for cities that purify stormwater for use as drinking water.

Stormwater, by definition, is the water from rain or melting snow that fails to percolate through the soil or get soaked up by vegetation. Instead, it moves across the surface of the ground - much of it streets, sidewalks, and parking lots - and pours into storm sewers. Occasionally it overwhelms the drains and causes flooding, unleashing a wealth of problems: waterlogged homes and businesses, polluted drinking water, and ruined habitat for wildlife, which in turn diminishes those populations. And because some localities co-mingle their stormwater and wastewater, when the system overflows sewage winds up in rivers and streams.

The sedimentation associated with soil erosion causes up to $7 billion in damage annually, destroying fish habitat and falling in channels and reservoirs.

SO WHAT CAN TREES DO IN THE FACE OF THAT TYPE OF ONSLAUGHT? PLENTY.

In one respect a tree is like a huge straw: It draws water through its roots and facilitates evaporation through its leaves. The physical barriers it provides - its roots and fallen branches - regulate the flow of runoff, reducing the water's speed and spreading out its flow. (The more concentrated the flow, the more it causes rutting and soil erosion.)

A tree's impact begins at its crown and continues on down to its roots. The canopy of leaves catches and holds rain and snow, reducing the volume of water that reaches the ground. Tree roots also stabilize soil, which prevents erosion upstream, and break up hard-packed soft, which allows it to absorb more water and prevents sediment deposits downstream.

Also, because infiltration - not runoff - is the primary source of streamwater, it's important to maximize permeation, in part through tree and vegetation cover in watersheds.

Most of the research scientists have done on the stabilizing effects of trees has come from studying tree loss. In the Chesapeake Bay's watershed, for example, deforestation has turned large storms into record-breaking floods, severely damaging towns and cities in the Bay area and dumping enormous amounts of pollution and sediments into the Bay, the nation's largest estuary.

A rapidly growing population is adding to these deforestation problems; the population is expected to grow by more than 1 million by 2000 and another 1.6 million by the year 2020 (an increase of 19 percent). Development associated with the expanding population is expected to eat up more than 600,000 acres of forest by 2020, increasing the amount of developed land by more than 14 percent, according o studies.

Flood problems are not limited to the East Coast, of course. A one-two punch of logging combined with heavy rains has resulted in worst-ever flooding in Washington's Skokomish Valley. The hard-hit Valley, part of the Olympic Peninsula and no stranger to flooding, recorded its worst ones in 1994, 1991, and 1993, respectively. Scientists are attributing that, in part, to heavier-than-usual rains, logging, and urban sprawl that has reduced the number of treed acres serving as natural buffers for stormwater. In addition, the construction of dikes and dams have blocked or diverted natural drainage systems and concentrated flooding in a few rivers or tributaries.

Nationally, floods caused an average $3.1 billion in damages annually between 1985 and 1995, according to the U.S. Geological Survey and the National Weather Service, and the problem appears to be worse now than ever before.

That's especially true in urban areas. Water quality standards set in the 1970s as part of the Clean Water Act were relaxed somewhat during the 1980s, but large cities still have difficulty meeting them. Atlanta, for example, pays $25,000 a day in fines for its failure to comply. A study conducted by AMERICAN FORESTS showed that the poor quality of the city's water was directly related to runoff from land development. The finding is part of an effort by AMERICAN FORESTS, together with federal and state partners, to develop a practical way to map Atlanta's ecology and analyze the benefits trees provide (see Atlanta's Changing Environment, Spring 1996).