Natural Valley Storage in the Charles River Watershed

Anne Pearce and Tom Pearce


The Charles River begins at Echo Lake in Hopkinton, Massachusetts. Flowing through 23 cities and towns over the course of 80 miles, the Charles dumps into Boston Harbor, passing MIT and Fenway Park near the end of its journey. The watershed encompasses 308 square miles, which covers 35 municipalities. This is the most densely populated watershed in all of New England.


With European settlement of Massachusetts came dramatic changes to the watershed. Beginning in the mid-1600s, water was diverted from the river to power grist and log mills for burgeoning industries. Over time, the Charles was dammed in 20 separate places to control flow for industry and agriculture. As the population of Boston grew rapidly at the turn of the twentieth century, the river’s floodplain was converted from wetlands and forests to homes, farms, and cityscape.

By the 1890s, the lower Charles was an entirely polluted and degraded tidal estuary, surrounded by industries along the Cambridge and Boston shores that used the river for dumping wastes. In 1893 Charles Eliot, a landscape architect who studied under Frederick Law Olmstead, produced a plan for the newly formed Boston Metropolitan Park Commission that would reclaim the estuary as parkland and move industry back from shore. This vision progressed and by 1907, a dam near the mouth of the Charles created a new waterfront park with an esplanade to complete the Charles River Basin in 1908. Aside from providing an excellent riverfront park, the Basin project also intended to secure Boston, Cambridge, Watertown, Waltham, Brookline, and Newtown from floods on the Charles. Of course, the new water supply attracted development in these towns and by the 1960s, the Charles was severely polluted once again due to low flows and loss of natural cleansing mechanisms.

A postcard depicting the Charles River Esplanade, c. 1910s-1920s.

Most importantly, development in the watershed and dams along the river led to significantly increased flood impacts for Charles River communities in hurricane-induced events in 1938, 1955, and 1968. The Great Hurricane of 1938 brought intense rainfall to the region and all of southern Massachusetts faced tremendous losses. Over 500 deaths and 1,700 injuries were recorded in southern New England, with 8000 homes destroyed and 15,000 damaged. Over a two week period in 1955, hurricanes Connie and Diane brought 25 inches of rainfall to coastal Massachusetts. Winter of 1968 saw higher than average snow cover and lower than average temperatures extending into early March. A warm up and subsequent rains of 4-7 inches on March 12-13 brought on intense flooding in eastern Massachusetts, causing $35 million of losses in the region.

In response to the floods of 1938 and 1955, the Army Corps of Engineers New England Division (Corps) began a study in 1965 to assess options for flood protection in the Charles River watershed. The Corps initially planned to build a series of dams along the river, but a citizen group, the Charles River Watershed Association (CRWA) formed in 1965 to push back against the dams. The 1968 flooding allowed the Corps to observe how different parts of the watershed responded to and were affected by the flooding; at the same time, the CRWA was advocating for non-structural flood protection in the watershed. Eventually, the study recommended that the dam at the mouth of the river be rebuilt and that wetlands along the upper reaches of the river should be protected as “natural valley storage areas.”


Urbanization and floodplains

The positions of stream and river channels are constantly changing through erosion and deposition processes. As tributaries input water to the main river channel, the increased volume (and power) of the water further down the channel causes larger changes in the position of the main channel and its floodplains. Development along a river and within its watershed impacts the flow of the river in several ways. Increased impervious surfaces like concrete and asphalt limit infiltration of water where it falls, causing runoff to reach a river more quickly and at higher volumes than it would under undeveloped conditions. This, in turn, means that a higher volume of water will reach downstream portions of the channel. The increased volume of water, in combination with the river’s natural channel-forming processes also increases the width of the floodplain. When development occurs, properties that were once safely out of the floodplain may eventually be well within the floodplain and be vulnerable to flood damages.

Development in and near floodplains increases the width of the floodplain. Property that was once safely outside of the floodplain may eventually be well within the widened floodplain.

Flood protection

As watersheds become increasingly urbanized, people look to provide flood protection through structural and non-structural practices. To explore the impacts of development and different flood control practices on flooding, check out this interactive application from the University of Kentucky’s Earth and Environmental Sciences department. Structural flood protection practices include constructing levees or floodwalls to contain water in the river channel and prevent it from reaching the protected properties. However, structures can be costly to build and maintain and are sometimes insufficient to prevent floodwaters from damaging property. Non-structural flood prevention includes prohibiting development within a floodplain and using natural valley storage (NVS). NVS refers to the practice of using the wetlands adjacent to a river to temporarily store water that overflows the banks of the river.

Wetlands provide a wide range of benefits, including trapping and slowly releasing water.

Why wetlands?

Wetlands have an amazing capacity to absorb and store water, essentially acting like giant sponges. The benefits they provide are numerous, as Bill Nye explains in the video below. In terms of flood protection, wetlands are beneficial for a few reasons. During a storm event, wetlands trap and store water where it falls. This prevents it from contributing to the large volume of water reaching tributaries and the main river channel. After the water is trapped in the wetland, it is either infiltrated on-site or slowly released from the site. This slows down the contribution of water to the river’s main channel, thus reducing the peak flow of the river during a flood. The amount of water stored in wetlands instead of running into the river might be the difference between a damaging flood and no flooding at all.

Skip to 4:10 in the video if you’re interested in what Bill Nye the Science Guy has to say about wetlands and floods.

Watershed management

When a river floods or has the potential to flood, the impacts usually cross several jurisdictional borders. Individual jurisdictions can certainly mitigate flooding on their own with structural and non-structural practices. However because the actions of upstream jurisdictions impact the amount and timing of water that reaches downstream jurisdictions, mitigating floods on a place-by-place basis is not enough. Rather, flood mitigation should be performed at the watershed scale. By approaching mitigation in consideration of the entire watershed, stakeholders will be better able to identify opportunities for and barriers to flood mitigation at all points in the watershed. In the case of using wetlands for flood mitigation, it is important to be able to identify whether the location and extent of wetlands within the watershed will provide enough protection to prevent flooding downstream.


The Charles River watershed can be divided into three distinct basins, the upper, middle, and lower basins. The lower basin includes the city of Boston, and is the most urbanized. The middle basin is more suburban, and the upper basin is more rural. As mentioned earlier, the Charles River Natural Valley Storage project was the result of a flood control feasibility study conducted by the Army Corps of Engineers (Corps) beginning in 1965. While the study recommended upgrading a dam at the mouth of the river as a structural form of flood control in the highly developed lower basin, it also determined that protecting certain wetlands in the upper and middle basins was an appropriate form of flood control for those basins and to provide additional protection to the lower basin. The NVS project was first considered in the late 1960s and enacted between 1977 and 1983, and was based on decisions made with tools far different from today’s tools. However, the type of information considered and the process used to analyze the information can and should be modified to include modern datasets and tools, as NVS is still a viable option for non-structural flood control. Ultimately, the decision whether or not to use NVS comes down to ecological, social, and economic factors.

Ecological factors

Soils, topography, and vegetation of potential storage areas impacts their utility for flood mitigation. Highly permeable soils like sands and gravels are best, because they allow water to infiltrate quickly. Areas with clayey soils are not good candidates for NVS because the clay limits infiltration of water. Additionally, areas with steep topography do not work well for NVS because the landscape does not sufficiently slow down the water. Finally, heavily vegetated areas have more capacity to absorb and infiltrate water without problems of soil erosion, as compared to sparsely vegetated or developed areas. In the Charles River watershed, the soils, topography, and vegetation were and are favorable for NVS.

Once it was determined that natural storage was suitable for the watershed, the Corps set out to determine the location and extent of storage areas. Storage areas in the lower portions of the watershed have a greater individual impact, but the storage areas in the upper portions of the watershed can, taken together, still provide extensive flood protection. The Corps used aerial photos to identify the location and extent of storage areas, as well as the extent to which they were being lost (filled). The Corps also studied the hydrologic characteristics of the storage areas. They compared the projected damage at different flood stage levels under scenarios with the storage areas in tact and with the projected future loss of storage areas. In this case, because significant flood damages were likely and because the protection of NVS was shown to have an impact on hydrologic conditions, the strategy proved its potential for significant benefits.

Social factors

While the ecological factors in the Charles River watershed were favorable for NVS, the Corps also needed to consider whether social factors warranted the protection of the storage areas. At the time of the study, it was determined that there was significant development pressure in the watershed which could threaten unprotected storage areas. There was also interest in using the NVS areas for recreation, although this was a secondary consideration for the Corps. Finally, at the time of the Charles River watershed study, there were few regulatory protections in place for wetlands, so it was important for the Corps to be able to protect them through easements or purchase. In later years, after the passage of the Clean Water Act, which protects wetlands from dredge and fill activities, the Corps studied the potential for NVS in four other New England watersheds. None of these studies resulted in the use of NVS, partially because it was deemed that regulatory protections of wetlands and local zoning ordinances made it unnecessary for the Corps to invest in permanent protection of storage areas for flood control (Kousky, 2014).

Economic factors

As for any project, the Corps conducted a cost-benefit analysis (CBA) for flood protection options in the Charles River watershed, using ecological and social factors in addition to the economic costs of structural versus non-structural practices. It was determined that the structural alternative to using NVS in the upper and middle basins of the Charles River watershed would likely have consisted of a 55,000 acre reservoir with a complex system of walls and channels, all of which would have had high construction costs and environmental damage (U.S. Army Corps of Engineers, 1993). Ultimately, it cost the Corps about $9 million to protect over 8000 acres of natural storage areas between 1977 and 1983. For comparison, the construction of the dam at the mouth of the river cost $61.3 million.

CBAs were also conducted for the four studies considering NVS in other New England watersheds. Each of these studies used different valuation techniques from the Charles River study and from each other. While each project is unique, the lack of standardization of CBAs from one agency highlights just one aspect of the complexity of developing flood control projects. Another point of interest includes the role of regulations as a factor in CBAs. It was noted that some watersheds did not pursue NVS because of the assumption that regulations provided enough protection to wetlands. However, even with a national policy of no net loss of wetlands, there is strong evidence that wetland regulations are not successfully protecting wetlands. Even when wetlands are restored to compensate for wetland losses due to development, the change in the physical location of the wetlands renders severely limits the utility of the wetlands for flood control. Finally, all CBAs still struggle with the difficulty of quantifying ecosystem services in a way that accurately reflects the many benefits of an ecosystem in a valuation system that can be compared to the economic costs of implementing a project.

The Charles River Natural Valley Storage project protected wetlands along the upper reaches of the river in order to mitigate floods.


The Charles River watershed is perhaps the best-known location of a successful NVS project. The findings of the studies of four other New England watersheds suggest that NVS is no longer a feasible option, given the protection afforded to wetlands by regulations and the high cost of protecting land. However, non-structural flood control should be part of any community’s  set of practices for increasing resilience to flood damages. In addition, non-structural practices like NVS can provide multiple other benefits, including recreational opportunities, improved water quality, and wildlife habitat.

Communities within a watershed and watershed associations can join forces to determine whether NVS is a viable option in their watershed. Today, the availability of data online means that stakeholders can easily explore NVS as an option in their watershed. Online tools like NOAA’s C-CAP Land Cover Atlas allow anyone to explore the changes in land cover and wetlands in their watershed. More experienced GIS users can combine soils, topography, and land cover data to perform a suitability analysis that determines whether and where a watershed contains natural storage areas. The suitability analysis used in combination with USGS stream gauge data can help stakeholders prioritize areas for protection. If time and funding resources allow, stakeholders within the watershed can conduct a more detailed study, in particular to perform hydrologic assessments of individual storage areas.

Beyond NVS, there are several other options for building resilience to floods in developed watersheds. Boston is known for Olmsted’s Emerald Necklace of green space around the city and for early efforts to maintain open space in the Charles River floodplain (such as the Esplanade). In an effort to continue to protect the Charles River, and perhaps with a nod to earlier visions of green space in the city, the CRWA is currently working on a Blue Cities Initiative. The initiative is aimed at restoring hydrologic function in urban watersheds by converting impervious surfaces to greenways. The group continues to focus on large scale projects, connecting several communities in other Massachusetts watersheds through restorations, urban green infrastructure retrofits, and the civic engagement and planning process behind these collaborations.

The CRWA is also developing a web-based stormwater market trading platform for pollution reductions. The Blue Cities Exchange simplifies the concept of stormwater trading by providing estimates for property evaluation and stormwater cost, as well as the costs of various green infrastructure practices or retrofits needed to manage pollution levels. The CRWA believes they will be able to roll out this web interface within the next 5 years.

As opportunities present, the CRWA continues to work on restoration projects, such as the 3.3 acre landfill at Medfield State Hospital, completed in 2015 and now called the Charles River Gateway. The CRWA also consulted with the City of Boston in 2013 to design green infrastructure principles and guidelines for the city’s  Complete Streets Policy and zoning code. All street redevelopments in the city now incorporate on-site stormwater storage and infiltration design and engineering.

Communities around the nation hope to increase resilience to flooding and to improve water quality in their watersheds. As climate change and development combine to alter the hydrologic characteristics and associated hazards within watersheds, communities will not be successful if they attempt to act alone to protect themselves. Rather, the watershed-scale collaboration and activities exemplified by the CRWA offers the best chance for solutions that increase resilience to natural hazards and provide other benefits to the communities.

Charles River Natural Valley Storage Area
Aerial view of the Charles River Natural Valley Storage Area in Massachusetts.


Charles River Watershed Association. “Charles River History.” Accessed 4/21/16 from

Kousky, C. 2014. The economics and politics of “green” flood control. Resources for the Future: Washington, DC.

National Weather Service. “Flooding in Massachusetts.” Accessed 4/24/16 at

Petersen, Mervin S., U.S. Geological Survey.“Floods of August 1955 in the Northeastern States.” Accessed 4/24/16 from

U.S. Army Corps of Engineers New England Division. 1993. Massachusetts natural valley storage investigation- Section 22 study.

Image and Video Sources

(In order of appearance)

Map of Charles River Watershed

Original Vintage Postcard: Esplanade from West Boston Bridge Boston, Massachusetts. Detroit Publishing Co.

Floodplain Urbanization

How Wetlands Work – Fabulous Wetlands with Bill Nye the Science Guy (1989) (

Map of Charles River Natural Valley Storage Areas

Aerial photo of Natural Valley Storage Area









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