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.

Mitigation and Adaptation Strategy: Conserve and Manage Forests 

An important set of Nature-Based Solutions relates to forests. Forests absorb stormwater and slow runoff. They provide us with clean water, clean air, biodiversity, forest products, and recreational opportunities. They are a critical component of our ecosystem. Additionally, they remove (sequester) carbon from the atmosphere and store it (above and below ground). Massachusetts is currently 62% forest cover, the 8th most forested state in the US and the 3rd most densely populated state.  However, forest cover in Massachusetts is decreasing and this means our environment's quality is being diminished and that carbon is being lost to the atmosphere.

Image left:  Above-ground storage of carbon. The Northeast and Northwest have the highest above-and below-ground carbon sequestration in the United States.  (https://www.nrs.fs.fed.us/carbon/tools/#cole) 

Forest ownership in the watershed includes private landowners, land trusts, and local, state and federal governments. The many types of owners have a wide range of goals for their forested land, such as income generation, recreation, ecology, and protection of water supplies or other natural resources. From the perspective of climate change, independent of other goals, maximizing the amount of forested land and managing forests for carbon storage and biodiversity are critically important. 

• The most important forest mitigation strategy is to stop the conversion of forest land to non-forest land uses and begin increasing forest cover. All forests sequester and store carbon, have significant ecological value, and provide numerous additional environmental benefits. Maximizing forest cover is critical to addressing the core causes and impacts of climate change.
• Non-managed, or minimally managed, forests generally provide the highest amount of carbon storage and highest ecological value. Older trees tend to provide the most carbon storage and forests with old-growth characteristics, such as a diversity of species, ages and size of trees, the presence of snags, large downed logs, and scattered canopy gaps provide especially valuable habitat for a wide range of animals. Although some younger forests may remove carbon from the atmosphere more rapidly than old-growth forests, the net effect of harvesting is a reduction in stored carbon.  
• When landowner goals for forests include income or other objectives, managing the forests to also preserve and enhance some old growth characteristics can retain extra carbon storage and reduce ecological losses.
• Educating forest owners about management options is essential to ensure that the best solutions are utilized to meet owner needs and ensure carbon storage and biodiversity.
• Wildlands and Woodlands, a science-based conservation vision for the New England Landscape developed as a collaboration among forestry organizations and conservation groups in New England, sets a goal of permanently protecting a minimum of 70% of the total land in New England as forests to be sustainably managed for timber harvesting and other values, plus at least 7% conserved as farmland. Additionally, Wildlands and Woodlands recommends 10% of the permanently protected forests be conserved as wildlands to protect biodiversity and wilderness. While those goals may be low, they do express the right directions to be moving. The NRWA is a collaborator in the Wildlands and Woodlands Partnership.
• The Forest Legacy Program, a Federal program in which the NRWA has been a collaborator, has also played a significant role in conserving forests. Read more about NRWA's Forest Legacy projects.

An Example: Restoring Old Growth Characteristics

Numerous scientific studies point to the importance of forests in storing carbon. Old growth forests play a role as carbon sinks as they continue to accumulate carbon in their wood and their soils actively capture carbon. Forests with old growth characteristics also provide the highest level of landscape diversity and ecological value. 

For managed forests, including smaller woodlots, such as the typical landowner is more likely to own and manage, the opportunity exists to restore old growth characteristics as a natural solution to climate change through modern forestry practices such as:

• leave “legacy” trees and very large trees 25+ inches diameter; 
• leave large standing dead trees; 
• leave large downed logs; 
• allow for long-term accumulation of coarse woody material; 
• support multi-aged trees and a variety of species; 
• allow and create gaps in the canopy; and
• limit soil disturbance during forest management.

Educating owners of large and small forests that their trees contribute to climate resilience, that “legacy trees” are particularly important for their role in carbon storage, and about the ways that targeted forest management practices mitigate climate change is essential.

Table and image above adapted from UMass Amherst Outreach Extension’s “Restoring Old Growth Characteristics” by Paul Catanzaro and Anthony D’Amato.

Mitigation and Adaptation Strategy: Protect and Enhance Floodplains, Wetlands, and Small Streams

Floodplains are nature's system for reducing the severity of floods. Floodplains are also critical open space and wildlife habitat since they are typically part of greenways along river and stream systems. Altered seasonal water flows may threaten flood plain functionality, which can adversely impact biological and nutrient cycles, and also seed dispersal. In addition, it may favor non-native over native species. Where the connections between floodplains and rivers have been severed, as by development, efforts should be made to reconnect them.

Wetlands and small streams provide flood storage capacity. They are also vital parts of our ecosystem, especially for many locally endangered species. They purify water and slow runoff from storm events. Moreover, they are also effectively sequestering and storing carbon. When wetlands are lost or degraded, significant quantities of carbon can be released into the atmosphere.

As stated by A. M. Nahlik and M. S. Fennessy (2016): "Soil carbon is vital in regulating climate, water supplies and biodiversity—all essential contributions to the provision of ecosystem services. Wetlands contain a disproportionate amount of the earth’s total soil carbon; holding between 20 and 30% of the estimated global soil carbon despite occupying 5-8% of its land surface. The anoxic conditions characteristic of wetland soils slow decomposition and lead to the accumulation of organic matter. As a result, wetlands can accumulate large carbon stores, making them an important sink for atmospheric carbon dioxide."

Ponds capture significant amounts of leaves and other organic debris. Similar to wetlands, anoxic conditions on the pond bottom slow decay and retain carbon. Ponds also temporarily store excess stormwater, reducing flows in streams and therefore down-gradient flooding.