Climate Change Impacts on Water in the Nashua River Watershed

Between 1958 and 2012, the Northeast saw more than a 70% increase in the amount of rainfall measured during heavy precipitation events—more than in any other region in the United States. Projections indicate continuing increases in precipitation. Rainfall events are increasing in both frequency and intensity, with most increases occurring in the winter and early spring when the ground cannot absorb water. This causes flooding and stresses the stormwater infrastructure. Another factor impacting the region’s hydrology is the steadily rising temperatures. This is causing an increase in the temperature of water in our wetlands, streams, ponds, and rivers, which will have a significant impact on their ecological value.

Although the rivers and major streams in our watershed form a relatively simple hydrologic pattern, many of the smaller streams and wetlands that drain into those waterways are unusually complex. The last glacier deposited or sculpted many of the landforms found throughout the watershed, including drumlins and outwash features such as eskers and kames. As a result, natural drainage paths were blocked, which gave rise to the extensive wetlands, ponds, and meandering streams characteristic of this area. This hydrologic system has enriched the ecological value of the watershed. However, this system does not efficiently drain floodwaters, thereby making portions of the watershed at increased risk for flooding.

Major Impacts on Water

The major hydrological impacts of climate in the Nashua River watershed include the following:

Flooding

The increased frequency and intensity of storms is causing a corresponding increase in periodic major flooding as well as frequent minor flooding of small streams and wetlands. Since bridges, storm drains, and other stormwater infrastructures were designed for typical historic storms, larger more intense storms in the future may overwhelm them. If a stormwater structure becomes inadequate, increased flooding will take place upstream of the structure, and may also result in bridge failures and road wash-outs. Significant disruption of travel and risk to emergency vehicles and personnel may result. Increased flooding also causes increased property damage and general disruption.

Erosion/Sedimentation

The flooding and increased flow rates in rivers and streams will cause increased erosion of stream banks and scour of river-bottom sediments. Land areas with sloping topography or thin vegetative cover will also be subject to increased erosion. Streams into which runoff from sloping areas drain will see increases in sedimentation. Portions of rivers that are wide or deep will see increases in sedimentation. These alterations to fluvial geomorphology further affect flooding potential and aquatic habitat.

"Flash" Droughts

The overall increase in total yearly rainfall should result in a decrease in the frequency of long-term droughts, but this is offset by predicted significant summer periods with little or no rain. This lack of rain, combined with a significant increase in the number of days with very high temperatures, will cause relatively shorter but particularly intense periods of drought, called Flash Droughts. These intense periods can cause small streams and wetlands to rapidly dry up, resulting in disruption to some of the watershed's most critical and sensitive ecosystems, including systems that support endangered and vulnerable species. Flash droughts will also cause lakes to become stagnant more frequently and to develop algal blooms.

Warming Water Temperatures

Many aquatic species are particularly sensitive to changes in stream, pond, or lake temperatures. Waterways that currently support important coldwater fisheries, including species of trout, may lose these species entirely. Warming water temperatures and extended summer periods will cause increased frequency and intensity of algae blooms. The rate of reproduction for most species of algae increases proportionately to, in some cases geometrically, with increasing temperature. In addition, algae blooms will raise the pH level, lower dissolved oxygen, and decrease light penetration, which will adversely affect the aquatic habitat of lakes and streams. Changing the aquatic habitat may favor non-native species over native species, thereby altering the structure and function of aquatic communities, as well as increasing the stress on native species.

Water Quality

Multiple adverse impacts on water quality are anticipated due to climate change. Warming water temperatures may mean the loss of some coldwater fisheries in the watershed. Higher total rainfall and more severe storms may increase suspended solids and turbidity due to scouring of streambeds and erosion of their banks. More acid rain can be expected, lowering the pH of the water and increasing stress on aquatic life. Increased loadings from non-point source pollution, including pollution from more distant sources can be expected. This may include increased nutrients, pesticides, and bacteria from rural areas; metals, oil and grease from urban areas; and salt from highways. Older cities with combined sewer systems can expect more frequent overflows to the river bringing increased turbidity and bacteria. Lower summer flows and less dilution will make point sources of pollution more pronounced. The combination of warmer water temperatures and increased pollution will lower dissolved oxygen levels that are necessary to maintain aquatic life.