How to interpret the data

Massachusetts Standards

* MADEP 1997 Massachusetts Surface Water Quality Standards—314 CMR 4.00 1997.
** MADEP 2002 Massachusetts Year 2002 Integrated List of Waters, Part 1.

Reference Conditions for Ecoregion XIV Subregion 59 Streams*

* adapted from USEPA. 2000. Ambient Water Quality Criteria Recommendations: Rivers and Streams in Nutrient Ecoregion XVI. EPA 822-B-00-022. United States Environmental Protection Agency: Office of Water, Office of Science and Technology, Health and Ecological Criteria Division. Washington, D.C. December 2000.

How much flow is enough to support a healthy population of fish?

Streamflow changes naturally over the course of a year. The highest monthly flows in New England occur with the spring runoff, then drop over the growing season to their lowest between late July and mid-September. Flows in each stream and at each point along the stream depend on the size of the watershed being drained, precipitation, the type of underlying geology, and alterations to the stream, such as dams and water withdrawals. To determine appropriate streamflow requirements, we need to understand the natural range of flows in a stream and the relationship between streamflow and habitat availability.

As a part of their larger regional effort to develop simple, accurate methods for evaluating healthy streamflows, USGS conducted R2Cross and Wetted-Perimeter method analyses on seven Assabet tributary sites. The results of these analyses, along with an analysis of long-term flow records from the USGS Nashoba Brook stream gauging site, are the basis for the streamflow scoring curves for the Stream Health Index:

• Assabet and Charles Rivers (2002)

• Assabet and Charles Rivers (2004)

More about R2Cross:

The shallow riffles and stream margins, which are important to fish habitat, are also the areas most vulnerable to drying out during low-flow periods. The R2Cross method is based on the assumption that flow sufficient to maintain habitat in a critical riffle will also maintain fish habitat in nearby pools and runs for most life stages. Data collection for the R2Cross involves in-stream measurements of stream geometry at a selected critical riffle, streamflow measurements at a nearby staff gauge, and measurements of the height of water (stage) over the riffle at a range of streamflows. A computer model (HEC-RAS) is used to predict the stage over the riffle over a full range of flows. Flow recommendations are then based on the flows calculated to maintain a recommended mean depth, percent wetted perimeter, and average velocity over the riffle.

Current thinking

Scientific understanding of streamflow requirements for habitat is continuing to evolve, and many great resources are available to dig further into this issue. Here are a few to start:

• Massachusetts Instream Flow Stewards

• Rushing Rivers Institute

OARS developed the “Stream Health Index” for the Sudbury, Assabet, and Concord river tributaries in 2002 to assess and communicate their conditions. An index brings information from multiple sources into a single number, like a grade, that can be understood at a glance.

This index compares current conditions with ideal summer streamflow, water quality, and habitat conditions for indicator fish at the sampling site in each stream. In setting out to evaluate habitat conditions for fish, we’ve assumed that protecting riverine fish and other aquatic wildlife is a worthwhile goal, and we’ve had to make some assumptions about the parameters that affect fish habitat. There are certain limitations to the interpretation of the index:

• The index does not reflect transient events affecting water quality or quantity
• The index does not rate the fish for consumption
• Toxic materials, viruses, and bacteria are not tested for

Overview

The Stream Health index is arranged so that environmental quality increases with the increasing index (I=1 for worst conditions; I=100 for best conditions). The table below displays the environmental parameters used and the criteria against which they are scored.

Calculation of the index consists of three steps

1. Calculate subindex scores for each of the parameters
2. Aggregate subindex scores into index scores for flow, water quality, and habitat availability
3. Aggregate the three index scores into the overall “Stream Health” index

Stream Health Index Parameters

Step 1

Calculating the individual scores: Water quality metrics are generally related to an index score along a rating curve. The Stream Health Index uses a combination of the Massachusetts Water Quality Standards, regional averages, and the indicator fish’s biological tolerances to develop curves for the water quality and flow parameters.

Below are sample curves developed for streamflow in Danforth Brook and dissolved oxygen. For example, if the streamflow in Danforth Brook (a cold-water stream) were 1.7 cubic feet per second (cfs) and the dissolved oxygen concentration was 6.5 mg/L on a given date, then the subindex scores for streamflow and DO would be 50 and 64, respectively.

Steps 2 and 3

Calculating the subindices and overall index: The trick in calculating an index is to combine the scores of the individual parameters in such a way that the overall index:

1. Gives a good picture of the overall score of all of the subindices
2. The most important subindices (for fish habitat) have the most influence over the overall index
3. The averaging process does not hide any low-scoring subindex

For this index, the subindex scores within each group are combined using a harmonic mean to get the index score. (For example, the harmonic mean of the streamflow and the groundwater level scores give the flow subindex.) Finally, the three index scores are combined by calculating the harmonic mean to get the overall Stream Health index score.

Using the harmonic mean (rather than an average or median) allows any low-scoring parameter to influence its subindex more heavily and any low-scoring subindex to influence the overall index more heavily. For example, if water quality and flow conditions were all good except for dissolved oxygen, which was too low for fish to survive, the overall index should be low.

Other Indices

While this is the first time (that we are aware) that an index like this one has been used to report habitat conditions on a weekly basis, our index follows in the footsteps of many previous water quality indices. A quick literature review shows that much of the initial work on water quality indices was done in the ’70s and early ’80s. However, interest is reviving with indices now in use in Washington, Oregon, Ohio, Colorado, Kansas, Texas, and British Columbia.