U.S. Geological Survey

U. S. Department of the Interior

USGS Fact Sheet 2004-3001v.1.01

May 2004

Introduction

Streamflow statistics such as 

flood peak flows, monthly means 

and medians, and 7-day 10-year low 

flows (7Q10) are used for a variety 

of purposes, including the design of 

bridges, culverts, and flood-control 

structures; the management and 

regulation of flood plains and reservoirs; and the allocation of limited 

water resources during seasonal low 

flows and droughts.

Although a number of rivers 

in Maine have long-term continuous 

gaging stations with up to 100 years 

of streamflow record, it often is 

necessary for hydrologists and 

water-resource managers to estimate flows on ungaged rivers, at 

ungaged locations on gaged rivers, 

or on gaged rivers with insufficient 

periods of record. To provide simple 

methods to estimate flow statistics 

at ungaged sites in Maine, the U.S. 

Geological Survey (USGS), in 

cooperation with State, local, and 

other Federal agencies, has developed methods that can be used by 

hydrologists, engineers, and 

managers to estimate characteristic 

flows based on measured hydrologic 

and climatic basin characteristics 

such as drainage area, wetland area, 

precipitation, elevation, and surficial geology.

Peak Streamflows

Regression equations to estimate peak flows with 2- to 500-year 

recurrence intervals in basins with 

drainage areas from 1 to 1,650 

square miles are presented in U.S. 

Geological Survey Water-Resources 

Investigations Report 99-4008 

(Hodgkins, 1999). Guidelines 

provided in this report assist waterresource managers in choosing the 

most appropriate equation to apply. 

Guideline criteria include evaluating 

the amount of development in the 

basin, the degree of regulation at the 

site of interest, the proximity of the 

site of interest to a gaging station, 

Methods for Estimating Streamflow Statistics for Ungaged Streams in Maine 

Figure 1. Flood on the Kennebec River at Augusta, Maine, April 1987.

Streamflow statistic

Basin size

0 to 1 

square miles

1 to 10 

square miles

10-1,500 

square miles

Peak flow Under development Hodgkins (1999) Hodgkins (1999)

Annual mean/median Not available Not available Dudley (2004)

Monthly mean/median Not available Not available Dudley (2004)

August median—Aroostook County Lombard and others (2003) Lombard and others (2003) Dudley (2004)

August median—Downeast Under development Under development Dudley (2004)

7Q10—Statewide Not available Not available Dudley (2004)

Flow duration Not available Not available Not available

Table 1. Methods for estimating streamflow statistics for ungaged basins by basin size.
and the degree of accuracy 

compared to the ease of application. 

Statewide equations are derived to 

calculate the 2-, 5-, 10-, 50-, 100- 

and 500-year recurrence interval 

peak flows based on the drainage 

area of the basin and the percentage 

of wetlands in the basin. A simplified technique using only drainage 

area is quicker and easier to apply, 

but results in estimates of peak 

flows that are less accurate.

Estimating peak flows for 

small basins (less than 1 square 

mile) often is done with models such 

as the Rational method and the 

National Resources Conservation 

Service TR-20 model. A crest-stage 

gaging network currently is in operation on 15 small basins throughout 

Maine, which will allow the accuracy of the models to be evaluated.

Mid-Range Streamflows

The USGS recently developed improved statewide regression 

equations to estimate annual and 

monthly mean and median streamflows at ungaged streams with 

drainage areas from 10 to 1,420 

square miles (Dudley, 2004). The 

new equations supersede those 

derived by the USGS 25 years ago 

(Parker, 1978). The new equations 

took advantage of 25 years of additional flow data and basin characteristics calculated with a Geographic 

Information System (GIS).

The statewide regression 

equations estimate annual and 

monthly mean and median flows 

using drainage basin characteristics 

including drainage area, surficial 

geology, precipitation, and distance 

from the coast.

Low Streamflows

A method to estimate the 

August median flow for ungaged 

basins in eastern Aroostook County 

is described in U.S. Geological 

Survey Water-Resources Investigations Report 03-4225 (Lombard and 

others, 2003). New generalized 

least-squares regression equations 

are applicable to small drainage 

basins from 1 to 40 square miles. 

Basin characteristics, including 

drainage area and mean elevation, 

are used to estimate August median 

flow. A similar study for small 

drainage basins in Washington and 

Hancock Counties currently is 

underway. 

New regression equations to 

estimate the 7Q10 low-flow statistic 

and summer monthly means and 

medians on drainage basins >10 

mi2

. also have recently been developed (Dudley, 2004).

Future Directions

In U.S. Geological Survey 

Water-Resources Investigations 

Report 99-4008 (1999), Hodgkins 

indicated that very steep watersheds 

may behave differently than other 

watersheds with regards to peak 

flows. Work to investigate peak 

flows on steep watersheds has been 

proposed.

References

Dudley, R.W., 2004, Estimating 

monthly, annual, and low 7-day, 

10-year streamflows for ungaged 

rivers in Maine: U.S. Geological 

Survey Scientific Investigations 

Report 2004-5026, 22 p.

 Hodgkins, G.A., 1999, Estimating 

the magnitude of peak flows for 

streams in Maine for selected 

recurrence intervals: U.S. Geological Survey Water-Resources 

Investigations Report 99-4008, 

45 p.

Lombard, P.J., Tasker, G. D., and 

Nielsen, M.G., 2003, August 

median streamflow on ungaged 

streams in eastern Aroostook 

County, Maine: U.S. Geological 

Survey Water-Resources Investigations Report, 03-4225, 20 p.

Parker, G.W., 1978, Methods for 

determining selected flow characteristics for streams in Maine: 

U.S. Geological Survey OpenFile Report 78-871, 25 p.

-by P.J. Lombard

Figure 2. 100-year drought at Piscataquis River at Dover-Foxcroft, Maine, August 2001.

For further information, contact:

District Chief

U.S. Geological Survey

196 Whitten Road

Augusta, ME 04330

(207) 622-8201