Wednesday, March 1, 2023

Watersheds and Land Use

The Occoquan watershed is often described as the most urbanized watershed in the nation. Think about that for a minute, certainly there are far more urbanized areas in the United States, but they do not have functioning watersheds. During their growth and development cities across the nation from New York, to Philadelphia through Baltimore and Washington, Chicago, San Francisco, Los Angeles and  hundreds more across the nation confined and subsumed many thousands of streams, erasing them from public memory.

In a study done early this century they found that Philadelphia had buried 73% of its streams. Another study counted 66% buried in Baltimore. Many streams that remained on the surface were sick or dying. A stream is a living ecosystem. It includes not just the water coursing between the banks but the earth, life and water around and under it. Beneath a living streambed is a layer of wet sediment, small stones and tiny living creatures called the hyporheic zone. Stream water filters down into this dynamic layer between surface water and groundwater, mixing with the groundwater pushing up to feed the rivers during dry spells. Water in the hyporheic zone flows downstream like the surface water above it but much, much more slowly.

For a large river the hyporheic zone is the essential engine of life. This zone can be tens of feet deep and can extend up to a mile laterally beyond the banks. It keeps the waterway healthy by regulating critical physical, biological and chemical processes, including riverbed aeration, water oxygenation, temperature moderation, pollution cleanup and food creation. Developing this lateral zone slowly begins the destruction of the watershed.

Cutting down streamside vegetation and woodland buffers that once slowed and absorbed rains causes floods. These floods were only made worse by covering the ground with compacted and/or impervious surfaces.  The faster flow of storm water gouged the riverbed. Later, urban planners and engineers funneled streams into buried pipes so they could build more city on top, disconnecting waterways from soil, plants and animals. The cumulative impact of these injuries led to flash floods, unstable banks, heavy pollution and waning life. 

In general, streams in urbanized areas are likely to have higher levels of oxygen demand, nutrients, suspended solids, ammonium, hydrocarbons, and metals. The negative impacts of urban land use on adjacent reservoirs, streams, and rivers have been well-documented in the literature. To establish effective water quality management policies, it is essential to understand the true nature of the relationship between water quality and urban land use.

Scientists have found that land use management can enhance or destroy stream water quality.  Particularly they found in a recent study when urban land use is in the range of 1.1%–31.5% of a watershed a watershed can still be restored. If urban land use exceeds 31.5% in a watershed water quality does not respond to restoration measures as expected. Once you destroy a watershed we do not know how to restore it.

Although close relationships between the water quality of streams and the types of land use within their watersheds have been well-documented, many aspects of these relationships remain unclear. Recent studies have suggested that the relationship is not linear which is commonly used in current land use models. Many of our remaining watersheds today are degraded characterized by degraded forests, invasive plants, soil erosion, erratic streamflow, declining groundwater resource, loss of biodiversity, microclimate deterioration, and declining ability to store water.

Prince William county is promoting regressive land use policies and practices, those from the last century when we did not recognize that all water is connected and the health of the watershed determines the quantity and quality of water resources. The hyporheic zone is an integral part of our freshwater ecosystems.  The hyporheic zone is made up of sediments, but is porous enough to allow the exchange of nutrients, dissolved oxygen, and water and serves to keep our streams healthy.  Water doesn’t just flow through the hyporheic zone, but has a specific residence time. This allows pollutants and nutrients to be removed, protecting water quality. During drought, flooding, and temperature extremes, the hyporheic zone becomes a refuge for many species. Let us hope that Prince William County can correct course before the Occoquan watershed is degraded beyond redemption. 

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