The Saco River Ecosystem

The Saco River

The Saco River originates in New Hampshire and terminates in Maine, in the United States. This watershed drains an area of 1,703 square miles (4,410 km2) of mostly forests and farmlands before discharging into the Atlantic Ocean at Saco Bay, 134 miles (216 km) from its source. It supplies drinking water to roughly 250,000 people in thirty-five towns. A 6-mile (10 km) long canal was created near Freyburg, Maine in the 1800s, shortening the river by 15 miles (24.1 km). The canal is now considered the official river channel since the original channel has been silted in. The collaborative dam database indicates that the Saco River Basin contains approximately 44 dams. Ten of the dams within the river basin are used for generating hydroelectric power, three are used for flood control and stormwater management, three are used for water supply, and fourteen are used for recreational use. Eleven dams are used for “other” purposes. The storage capacity of impoundments in the Saco River Basin is approximately 96,000 acre-feet. Of interest is the impact of dams on wildlife passage. Data on historical and current habitat of several fish species in Maine rivers including the Saco can be found here.

The Saco River is the fourth largest river (by discharge volume) in the State of Maine. Two of the larger rivers (the Androscoggin and the Kennebec) join before entering the Gulf of Maine, so the Saco River is actually the third largest point source of freshwater into the coastal ocean. Discharge volumes are highly variable, but can exceed 600 m3 s-1 during spring run-off events (as measured at the Florida Power and Light dam in Saco). In contrast to the three largest Maine rivers (the Androscoggin, Kennebec and Penobscot), the Saco enters the Gulf of Maine through a relatively narrow mouth (circa 100 m) flanked by two jetties, and so has a very discrete point of entry, which facilitates monitoring activities. 


The Saco River Basin contains all or portions of two cities (Saco and Biddeford), 29 towns, two unincorporated areas, and falls within three counties. Table 48 presents the historical population data within the Saco River Basin. The population within the drainage basin has increased since the 1970s, and the number of people residing in the cities continues to increase.  

Census Date          Population           Population in Cities
1970 38,900 20,600
1980 50,100 20,800
1990 59,200 22,600
2000 67,100 24,000

Saco Bay

The Hydrological Cycle

Average annual precipitation within the Saco River Basin is 44.8 inches  (113.8 cm) with a high proportion of rainfall occurring during November and December.  Annual snowfall ranges from forty inches (111.8 cm) near the Atlantic Coast to 115 inches (292 cm) in the mountainous headwaters. The water content of the snow averages five inches (12.7 cm) over the entire basin.  

Land use/Land Cover and Climate. Land use changes have been shown to alter regional climate and vegetation in adjacent natural areas, primarily through agriculture and urbanization (Stohlgren et al. 1998).The importance of land-use and land cover in climate change is related to a number of factors — changing surface albedo, effects on the hydrological cycle and alteration of biogeochemical cycles (Carpenter et al. 2007). Worldwide there has been an observed increase of river runoff (Labat et al. 2004, Gedney et. al. 2006). The cause of the increased runoff has been attributed to the variability of global climate (Piao et al. 2007).

River Discharge and the Coastal Ocean

The dynamics of the coastal ocean are inextricably linked to the terrestrial environment by rivers. River discharges affect coastal circulation patterns and may carry nutrients, sediment and contaminants that affect coastal food webs. Buoyant river plumes often extend over large areas and affect transport and mixing for many miles downstream. Biological processes in the coastal ocean are closely coupled to the dynamics of freshwater plumes, so understanding the effects of river plumes on the coastal environment requires understanding both physical circulation and mixing processes and how those physical factors affect water chemistry, nutrients, phytoplankton, zooplankton and eventually higher trophic levels. Because rainfall and snow melt are strongly seasonal and exhibit interannual variation, river plumes and the biological processes they influence are expected to be highly dynamic and vary on time scales of days, months, years and decades (e.g., Yanagi & Hino 2005, Thomas & Weatherbee 2006). Understanding the causes and consequences of temporal variation is particularly critical for predicting the impacts of climate change on coastal oceans.

Marine science has entered an era in which data collected by remotely deployed instruments are playing an ever increasing role in our understanding of physical transport and coupled biological processes. Remote monitoring programs are particularly essential for addressing issues related to climatic forcing of coupled biophysical systems. Low spatial resolution projects like the Hawaiian Ocean Times Series (HOTS) and Bermuda Atlantic Time Series (BATS) have provided new insight into open ocean temporal dynamics, while in coastal environments higher spatial resolution observing systems like Rutger’s LEO 15 and, locally, the Gulf of Maine Ocean Observing System (GoMOOS, now NERACOOS) have greatly expanded our understanding of temporal variation in spatial dynamics. As a reflection of the rapid increase in interest in observatory science, numerous large-scale ocean observing systems are either in place or under development, and significant federal funding is expected to flow into observing infrastructure (particularly through NOAA’s IOS and NSF’s ORION programs).

Historical Development

European settlement of North America has brought about many changes in the landscape including deforestation, agriculture and alteration of the flow régimes of rivers (Foster et al. 1998, 2003, Poff et al. 1997). The full extent of these modifications may not even be fully realized yet. For instance, Walter and Merritts (2008) show that streams in the Mid-atlantic states were small and anabranching channels in pre-colonial time. The streams are now deeply incised channels cut into fill-terrace deposits, found behind thousands of former milldams that replaced the pre-settlement wetlands. The landscape of New England has also been thoroughly modified over the past 350 years (Williams 1982). The progression of changes went from deforestation and agriculture by colonists to subsequent farm abandonment and natural reforestation, in addition to population expansion and urbanization (Foster 1995, 1998, Whitney 1994). In Maine, the historical modification of the Kennebec River and estuary complex has been documented to show deleterious effects of dam building and industrialization (Köster et al. 2007). The current vegetation pattern is compositionally distinct from Colonial vegetation and shows little relationship to broad climatic gradients (Foster et al. 1998). While there have been documented changes in land use, the implications of these changes are not known. 

Climate Linkages

Climate change will alter the linkages between the land and ocean. Over the past century, temperatures in Lewiston, Maine have risen 3.4 ºF, and are predicted to rise further (USEPA, 1998). The HadCM2 model predicts an increase in precipitation of 10% ± 5 in spring and summer, and an increase of 30% ± 20 in winter (USEPA, 1998). Precipitation increases would lead to higher stream flow in winter and spring. However, summer stream flows might be reduced because of increased temperature and evaporation. Increased runoff from streams would cause a seasonal freshening of coastal waters. Higher summer temperatures might also lead to decreased oxygen concentration in estuarine waters. Superimposed on climate change will be effects of the North Atlantic Oscillation (NAO; Hurrell 1995, Hurrell & Dickson 2004). During positive phases of NAO the eastern United States generally experiences mild winter conditions and reduced snow cover. NAO effects on terrestrial carbon and water cycles (D'Odorico et al. 2001) will in turn impact the coastal waters. Ecological effects of warmer ocean surface temperatures include a possible higher incidence of harmful algal blooms (Stenseth et al. 2002). The changes in climate associated with the NAO have also been implicated in major shifts in zooplankton biodiversity in the North Atlantic (Beaugrand et al. 2002), and at least partial control of right whale populations in the Gulf of Maine through effects on copepod abundance (Greene et al. 2003). Additionally, wave heights in the Northeast Atlantic are closely correlated to the winter NAO index (Bauer 2001), which would lead to changes in mixing.