How the Index is Calculated 

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Minimum Flow Requirements

How much is enough flow in a stream 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 an understanding of the natural range of flows in a stream and of the relationship between streamflow and habitat availability in that stream.

There are a variety of methods for determining streamflow requirements for habitat protection. USGS recently compared several methods in their work on the Ipswich River and on the Assabet and Charles Rivers. 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 of the StreamWatch monitoring sites. The results of these analyses, along with an analysis of long-term flow records from the USGS Nashoba Brook stream gaging site, are the basis for the streamflow scoring curves for the StreamWatch Stream Health Index.

More about R2Cross:

The shallow riffles and stream margins 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 gage, 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 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.

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