Low Flow Database

Measurement Methods

Currently there are no regulations for managing streamflow based on the biological needs of a river. However, there are a number of scientific methods available to determine how much water should be left in a river to support a healthy river community. Each method makes different assumptions about what is most important to aquatic communities. A few of the most common methods are:

Range of Variability (RVA) – This method was developed by the Nature Conservancy in response to the growing concern that natural variability and not just minimum streamflows are required to support healthy aquatic communities. Using RVA, 33 statistical aspects of streamflow are compiled and target streamflows are suggested to fall within one standard deviation of the mean values. For example, the duration of low flow periods should fall within one standard deviation of the historic duration of low flow periods.

Mesohabitat Simulation Model (MesoHABSIM) – Currently under development at the University of Massachusetts, Amherst, MesoHABSIM is based on the IFIM concept of habitat modelling (see below), but focuses on larger reaches, or "meso-habitats" such as riffles, runs and pools. Physical characteristics of each mesohabitat are measured and compared to fish habitat preferences to determine the amount of useable fish habitat at different water levels. MesoHABSIM is currently being refined on several watershed-scale assessment projects.

Instream Flow Incremental Methodology (IFIM) – IFIM was developed by the USGS as a way to quantify and manage the effects of streamflow on fish. First, a target fish species or several key species are identified. Next, a study of the types of habitat occupied by each life stage of each species is conducted in the river. Then, a relationship between different streamflows and the availability of each type of habitat is determined. Minimum flow recommendations are developed by weighing the amount of habitat available for each species and life stage at low flows.

New England Aquatic Baseflow (ABF) – This method was developed as an interim policy for minimum streamflows in New England by the US Fish and Wildlife Service. The August median flow is suggested as a minimum summer flow value because it represents the most severe naturally occurring condition that a stream community would experience. For ungauged streams, where the August median flow is unknown, a summer default value of 0.5 cubic feet per second per square mile of drainage area (cfsm) is suggested based on a survey of minimally impacted reference streams and rivers in New England. Because higher flows are needed at other times of the year for spawning, migration and other biological needs, USFWS’s minimum flow recommendation for fall is 1.0 cfsm and for winter and spring is 4.0 cfsm. Where a minimum of 25 years of data is available, monthly median flows are suggested.

R2Cross – A combination of wetted perimeter (see below), depth and water velocity is used to describe the minimum flow necessary to maintain acceptable habitat for fish and macroinvertebrates in critical areas such as riffles. By maintaining habitat in these critical areas, the model assumes that other habitats, such as pools and runs, will also be maintained.

Wetted Perimeter – The wetted perimeter (the submerged stream width) in riffles is used as an index of fish food availability. It is assumed that by maximizing the wetted perimeter of riffles, enough fish food and habitat will be available to support a healthy aquatic community in the river as a whole. The minimum streamflow required for habitat protection is the one where increases in streamflow no longer produce large increases in wetted perimeter. This point usually occurs when water covers the streambed to the bottom of the bank.

7Q10 – The 7Q10 refers to the lowest consecutive seven-day streamflow that is likely to occur in a ten-year period. It is used by many states and the federal government in setting discharge limits in National Pollutant Discharge Elimination System (NPDES) water quality permits. A permit will only be granted if the proposed amount of pollutant that will be discharged into a river will not significantly impair the designated uses, such as drinking or swimming, when the streamflow falls to the 7Q10 level. In other words, NPDES permit holders are restricted from discharging pollutants that would cause pollutant concentrations in the receiving water to exceed permit limits, even at very low (i.e., 7Q10) streamflow levels. Although such a low streamflow value, roughly equivalent to a ten-year drought, is appropriately used in the context of limiting pollution discharges, the 7Q10 flow statistic is sometimes inappropriately claimed to represent an adequate streamflow for maintaining a healthy aquatic ecosystem, when in fact much higher streamflow levels are required.