Water Quantity

Water Quantity


The NH Department of Environmental Services has been trying to adopt instream flow rules for over fifteen years. Similar to those in other states, these rules would be geared toward protecting existing instream water uses and insuring that sufficient aquatic habitat conditions are maintained throughout the range of seasonal flow conditions. As an interim measure, in 2002, the NH Legislature authorized and provided funding for NHDES to conduct Pilot Studies on the Souhegan and Lamprey Rivers to assess how the proposed methodology and study results would affect the existing water users. These studies are ongoing and are anticipated to be completed in 2009.

Instream flow generally pertains to the volume of water within a specific river segment or channel cross-section at any given time and has components of frequency of occurrence and duration. Stream flow is generally composed of groundwater inflow also known as base flow and surface runoff from adjacent land areas produced during storm events and snow melt. Stream flow is measured and expressed as a rate of flow in terms of cubic feet per second. Stream flow can be reduced by indirect and direct water withdrawals, altered by flow controls such as dams and by changes in the land uses within the watershed. Studies have shown that as the imperviousness of the watershed increases, more water enters the stream as direct runoff during or immediately after a rain event and less water infiltrates to the groundwater, which ultimately reduces the base flow that occurs between storm events. As a result, streams in urbanizing areas tend to be more “flashy” with much greater, short-term peak flows during storm events and lower base flows during dry weather. This shift to higher peak flows and lower base flows can result in dramatic changes in channel geometry, increase the flooding potential downstream and reduce the availability of water for other uses during dry weather conditions. Recent regulatory approaches (e.g., proposed AoT rule changes) are seeking to promote greater infiltration of site runoff for development projects to counter this effect.

Maintaining stream flow as close to its naturally occurring conditions is vitally important for maintaining healthy aquatic habitat conditions with respect to the depth of water, flow velocity, and width of stream channel that is inundated with water. Stream flow supports other public uses as well including navigation, recreation, fishing, conservation, wildlife habitat, protection of water quality and public health, pollution abatement, aesthetic beauty, public water supply, and hydropower production. Typically, a reduction in stream flow in terms of the rate of flow and duration has the greatest adverse affect on these uses. A reduction in stream flow can result from increased water withdrawals, dam modifications or less groundwater inflow or base flow due to increased runoff and less ground water infiltration (DES has prepared a Water Use Report for the Upper Merrimack River that is available via their web site in the instream flow rule section). Lower stream flow can diminish water quality conditions by causing higher water temperatures, reduced dissolved oxygen levels and a lower assimilative capacity for wastewater treatment. Lower stream flow volumes results in less dilution, causing higher concentrations of contaminants. Declining water quality and water depths can affect uses and the availability of water for public water supply, swimming, fishing, aquatic habitat as well as boating activities.

GOAL 3: Instream flows in the upper Merrimack River that support the public uses and the outstanding resources for which the river was designated such as habitat, recreation, wildlife, agriculture, wastewater treatment discharges, and drinking water.

OBJECTIVE: SF-1 – By end of 2008, identify all instream protected uses and outstanding characteristics and resources (IPUOCRs), as described in RSA 483 such as wastewater allocation flow, aquatic habitat, water supply, recreation needs, irrigation, and withdrawal will be identified.


  • Review draft documents from Souhegan and Lamprey River to evaluate methodology for determining the flow needs to support flow-dependent instream public uses, outstanding resources, and characteristics.
  • Identify existing water supply and irrigation withdrawals and quantify usage amounts.
  • Conduct visual surveys along corridor to identify, verify, and report any large (>140,000 gallons per week) water withdrawals.
  • Identify existing dams within entire watershed and determine their flow maintenance operations.
  • Evaluate the affects of maintaining summer water levels in major lakes and impoundments on downstream flow.
  • Identify wastewater discharge points and any flow allocation (i.e. 7Q10) contained in permits.
  • Identify available USGS gauging stations in the region and assess availability of historical flow records to develop flow duration curves similar to Souhegan and Lamprey studies.
  • Conduct a random citizen and/or stakeholder survey to obtain public feedback on the important issues and concerns in the river corridor (a similar survey was conducted for the Lamprey River by a UNH graduate student specializing in resource economics).


    • – By mid-2009, conduct a preliminary assessment of the flow volume needs to preserve and/or maintain the protected uses and outstanding characteristics and resources. Identify any possible flow management measures needed to address data gaps and/or estimated flow deficiencies (this objective is newly created to separate the hydrologic modeling or flow determination needs for each of the IPUOCRS as opposed to the simple identification of IPUOCR in the previous objective since the flow modeling efforts will require much more time (as was found in the Souhegan and Lamprey River studies).


      • Conduct in-stream surveys at key locations to evaluate habitat flow needs based on Pilot Study methodology.
      • Develop seasonal flow duration curves for key multi-use river segments using historical flow records.
      • Identify any data gaps in historical flow records and measures to bridge or fill-in gaps.
      • Quantify flow volume needs for each of the IPUOCRs including future water supply and waste water flow planning estimates and compare flow needs to the historical seasonal flow duration curves to identify flow deficiencies.
      • Determine amount of flow need to overcome estimated flow deficiencies.
      • Consult with NHDES Drinking Water and Ground Water Bureau to acquire future planning estimates of water supply and wastewater flow needs developed as part of the larger Merrimack River Watershed Project.
      • Continue to monitor and review re-licensing applications for dams in the upper Merrimack River corridor and participate as interveners with the Federal Energy Regulatory Commission (FERC) process.

OBJECTIVE: SF-3 – By end of 2009, create a plan that identifies and prioritizes existing flow alterations and water consumption issues and identifies sources of funding to study or implement corrective measures to reduce effects of water consumption and improve flows in the upper Merrimack River watershed.


    • Review proposed corrective actions, if any, in draft Souhegan and Lamprey River Plans and assess applicability for use in the upper Merrimack River reach.
    • Research and promote alternatives to water conservation practices for commercial and residential uses.
    • Evaluate water usage from local withdrawal sources on an average daily and seasonal demand basis as well as possible future demand and expansion plans and any conservation practices and/or policies that may be included in their operations.
    • Educate community residents and key stakeholders about the benefits of water conservation measures through educational mailings or presentations at local events.
    • Work with NHDES to identify funding sources needed to complete field efforts, promote education or implement corrective actions.
    • Coordinate and provide recommendations to NHDES on measures to improve flow management or reduce consumptive uses for the existing potentially affected stake holders as well as those for any future proposed uses or operational changes.