Bibliography Background About KRIS

Flow as a Limiting Factor for Atlantic Salmon in Maine DPS Rivers and Possible Solutions

Water flow levels in streams affect all aquatic life. The complex life history of Atlantic salmon has evolved with natural flow fluctuations of Maine coastal rivers and is tuned to the its home river for such things as spawn timing and smolt out-migration. Low flow conditions are recognized as potentially limiting to the remnant wild Atlantic salmon populations in the seven rivers recognized as a Distinct Population Segment (DPS) (MASTF, 1997). The National Marine Fisheries Service and U.S. Fish and Wildlife Service (2004) Draft Recovery Plan for the Gulf of Maine Distinct Population Segment of Atlantic Salmon lists "excessive or unregulated water withdrawal" as one of the reasons for the need to protect the species under the Endangered Species Act. They noted that "the potential impacts of water withdrawals from DPS rivers and streams include limiting summer habitat for parr, low winter flow effects on redds and egg incubation as well as adult immigration .... and smolt emigration."

The annual flow cycle for DPS rivers show the following seasonal trends (NMFS/USFWS, 2004): 

cascade.JPG (898251 bytes)

This photo of the West Branch Sheepscot River was taken during high spring flows, which typically result from snow melt. These flows trigger downstream migration of smolts and also allow kelts, spawned out adult Atlantic salmon, to return to the ocean. Dams may reduce this spring pulse and, thus, interfere with natural cues smolts need for optimal timing of their seaward migration. See picture of same location at base flows.




Long Term Maine Coastal River Basin Flow Trends

The U.S. Geologic Survey (Dudley and Hodgkins, 2002) studied data from several Maine coastal rivers to look at trends in flow, river ice formation, rain fall, and snow fall from the early 1900's to the present. The best available data was for 1938-2002. The study included the Dennys, Machias, East Machias, Narraguagus, Union, Pleasant, Royal and Sheepscot rivers. They found some significant trends:

Irland (1998) found that during the 20th Century, forest cover in the Sheepscot River watershed and other Maine river basins had increased by 100-186%, as active farming decreased. Dudley and Hodgkins (2002) note that previous studies suggest water yield should decrease with reforestation due to increased evapotranspiration, but their "results suggest an increase in precipitation and (or) a decrease in evapotranspiration" leading to increased Sheepscot basin water yield. The Maine Atlantic Salmon Commission (1997) states that "increased forest cover is better able to intercept and retain precipitation than an area with little ground cover." 

The National Research Council (2003) expressed concerns about global warming and potential challenges to maintaining Atlantic salmon in Maine: “Freshwater mortality…..may be amplified by direct and indirect effects of changes in precipitation, seasonal ice formation, temporal patterns of stream flow and other local properties associated with global climate change.” NRC opined that some degree of climate warming and hydrologic change could be tolerated by Atlantic salmon "if most of the other problems affecting Maine's salmon are reduced." Specifically they recommend improving riparian vegetation and changing watershed management to help restore the timing and volume of flows.

Effects of Flow on Atlantic Salmon Life History Phases

Adult Atlantic Salmon: Low flows may limit adult salmon entry into the river or movement up river to stage for spawning (NMFS/USFWS, 2004) and may increase stream warming.Halsted (2002) and Shepard (1995) found that high summer water temperatures in Maine rivers constrain adult salmon movements. Mills (1991) addressed the use of timed pulse flows for attracting adult Atlantic salmon migration into rivers, but found that to achieve the desired result, flows must occur in conjunction with other cues such as tides, onshore winds, cooler weather or natural freshets that might moderate water temperatures. Atlantic salmon adults may survive spawning and hold in freshwater throughout summer as "kelts", where they are subjected to same limiting factors as parr associated with low flow conditions. 

riffle.JPG (926959 bytes)This photograph shows a riffle habitat in a tributary of the Sheepscot River during low flow conditions. In dry years, as the water level drops, the stream becomes shallower and less suitable for Atlantic salmon parr rearing. Decreased stream velocity may reduce food delivery, making habitat less than optimal.

Parr: The NMFS and USFWS (2004) note the following about Atlantic salmon parr and stream flows: "Parr growth and survival during the summer are positively correlated with various flow rates, demonstrating that the low flows limit parr populations. Population reductions during low flows probably occur because of reduction in habitat quantity and quality and possibly reduced foraging opportunities (Frenette et al. 1984). This reduction in habitat quantity and quality can cause salmon parr to shift to sub-optimal habitat, reducing foraging opportunities and thereby impairing growth and survival." The Maine Agricultural Working Group (1995) noted that standing crops of juveniles might vary with flow but that the variable juvenile and adult life histories of Atlantic salmon tended to smooth out population swings caused by periodic low flow years. They noted, however, that "if annual summer flows are constantly low due to irrigation, the population size will be constrained by the available habitat at those flows and will not vary as greatly as when flows were unregulated. The carrying capacity of the river to produce juveniles will be reduced for the long term, not just for an occasional year." See Temperature and Atlantic Salmon Background page for more information.

Eggs/Alevin/Fry: Baum (1997) noted that fewer than 10% of Atlantic salmon eggs survive to emerge as fry in Maine rivers. NMFS and USFWS (2004) note that "sources of egg mortality include de-watering, freezing, mechanical destruction (i.e., sedimentation) and predation." They also describe relationships of flow and survival in the alevin and fry stages: "The timing of hatching and emergence, relative to spring runoff, affects egg to fry mortality and survival. Low flows in the thirty days prior to spring runoff may cause high mortality among pre-emergent alevins (Frenette et al. 1984). Unusually high spring runoff may scour redds, causing pre-emergent alevins to drift downstream prematurely. High flows within one week after emergence can cause fry mortality or displace fry to sub-optimal habitats" (Jensen and Johnsen 1999).

Maintaining Flows for Atlantic Salmon while Supplying Water for Agriculture

The Maine Agricultural Working Group (1995) suggested using well defined, scientifically based flow requirements associated with Atlantic salmon habitat downstream of large agricultural use in Total Water Management Plans for all DPS watersheds. This concept was embraced by NMFS and USFWS (2004) but rather but the name was changed to Water Use Management Plan (WUMP) and prioritized only for for Downeast rivers. The Draft Gulf of Maine Atlantic Salmon Recovery Plan (NMFS/USFWS, 2004) implementation schedule lists the following measures related to flow:

Water Use Management Plans have now been developed for the Pleasant and Narraguagus rivers and Mopang Stream, a tributary of the Machias River (NMFS/USFWS, 2004). These plans have called attention to the potential harm to Atlantic salmon caused by surface water withdrawal during low flow conditions. Consequently, almost all large agricultural operations are pumping ground water from wells in the summer. The Maine Atlantic Salmon Commission (1997) noted that base flows in summer were supported by cold ground water. NMFS and USFWS (2004) acknowledge that there are no studies of the connection of surface and ground waters in these basins and that potential negative effects on surface flow could result from ground water pumping.

Another solution to improving base flow conditions is the use of impoundments to save spring melt water for release during summer rearing or fall adult migration periods. Natural lakes in the headwaters of Maine coastal rivers were known to reduce spring peak flows and provide higher base flows in summer and early fall (MASC, 1997) and this strategy is based on that principal. Increasing storage for Atlantic salmon was explored on the Sheepscot River, but only Sheepscot Pond had the capacity for cold water storage and increasing its size was not considered feasible because of costs (Meister, 1982). Kircheis (2001, 2002) described studies on the Dennys River below Meddybemps Dam to determine optimal flow for Atlantic salmon and subsequent water release strategies that "optimize salmon habitat and optimally manage the water budget of the system."  When reservoirs are used for flow augmentation, the effects of capturing spring flows on the emigration of smolts needs to be evaluated (NMFS/USFWS, 2004).

Stream Flow and Urbaniztion

In areas of urban or suburban growth, developments should be designed to retain rather than drain stormwater, which will protect groundwater recharge and help reduce water quality problems. See the Urbanization and Atlantic Salmon Background page for more information.


Baum, E. (1997). Maine Atlantic Salmon: A National Treasure. Hermon, ME:, Atlantic Salmon Unlimited. 224 p. 

Dudley, R. W. and G. A. Hodgkins, 2002. Trends in streamflow, river ice, and snowpack for coastal river basins in Maine during the 20th century. U.S. Geological Survey. Water-Resources Investigations Report 02-4245, in cooperation with the Maine Atlantic Salmon Commission. Augusta, ME. 35 pp. [2.75Mb]

Frenette, M., M. Caron, P. Julien and R.J. Gibson 1984. Interaction entre le debit et les populations de tacons (Salmo salar) de la riviere Matamec, Quebec. Canadian Journal of Fisheries and Aquatic Sciences 41:954-963.

Halsted, M., 2002. Effects of stream flow on the stream temperature, E. coli concentrations and dissolved oxygen levels in the West Branch of the Sheepscot River. Alna, ME. 15 pp. [450kb]

Irland, L.C., 1998, Maine’s Forest Area, 1600-1995-Review of Available Estimates. College of Natural Sciences, Forestry, and Agriculture, University of Maine, Maine Agricultural and Forest Experiment Station. Miscellaneous Publication 736. 12 p.

Jensen, A.J., B.O. Johnsen 1999. The functional relationship between peak spring floods and survival and growth of juvenile Atlantic Salmon (Salmo salar) and Brown Trout (Salmo trutta). Functional Ecology. 6: 778-785.

Kircheis, F.W., 2001. Annual Report to the Maine Legislature Fish and Wildlife Committee for the period January through December 2001. Maine Atlantic Salmon Commission, August, Maine. 59 p. [1.4 Mb]

Kircheis, F.W., 2002. Annual Report to the Maine Legislature Fish and Wildlife Committee for the period January through December 2002. Maine Atlantic Salmon Commission, August, Maine. 73 p.[2.4 Mb]

Maine Agricultural Working Group. 1995. Agriculture and Atlantic Salmon: Response to ESA Proposed Listing of Atlantic Salmon. Report of the Agricultural Working Group to Governor Angus King, Augusta, ME. 84 p.

Maine Atlantic Salmon Task Force, 1997. Atlantic Salmon Conservation Plan for Seven Maine Rivers. 309 pp. [1.6Mb]

Meister, A. L., 1982. Sheepscot: An Atlantic salmon river management report. Atlantic Sea Run Salmon Commission. Bangor, ME. 47 pp. [3.7Mb]**

Mills, D. 1991. Ecology and Management of Atlantic salmon. Chapman and Hall, University Press, Cambridge, U.K. 351 p. 

National Marine Fisheries Service (NMFS) and U.S. Fish and Wildlife Service (USFWS). 2004. Draft Recovery Plan for the Gulf of Maine Distinct Population Segment of Atlantic Salmon (Salmo salar). National Oceanic and Atmospheric Administration, NMFS, and Northeastern Region USFWS. Silver Spring and Hadley, MA. 239 pp. [950 kb]

National Research Council, 2003. Atlantic Salmon in Maine. The Committee on Atlantic Salmon in Maine, Board on Environmental Studies and Toxicology, Ocean Studies Board, Division on Earth and Life Sciences. National Research Council of the National Academies. National Academy Press. Washington, D.C. 260 pp. [3.5Mb]**

Shepard, S.L. 1995. Atlantic salmon spawning migrations in the Penobscot River, Maine – Fishways, flows and high temperatures. MS Thesis, University of Maine, Orono, Maine. 111 pp.