13. Aquatic Connectivity |

At a Glance

12 dams have been removed in the Casco Bay watershed since 2002, including a summer 2026 project at Bridge Street Dam, the lowest dam on the Royal River.

Since 2000, through restoration actions, 150 miles of riverine habitat in the Presumpscot watershed have been reconnected to Casco Bay.

River herring counts have trended upward since 2002, when Smelt Hill Dam was removed at Presumpscot Falls. In 2026, an estimated 60,000 alewives swam from the Presumpscot Estuary into Highland Lake, to spawn.

Since the 2021 Report

Eight dams have been removed since the prior State of Casco Bay report in 2021, including two Presumpscot main stem dams at Saccarappa Falls in downtown Westbrook, where fishways were also constructed to aid movement around the industrially-modified falls. As a result, for the first time in centuries, blueback herring, shad, and other anadromous fish have access to an estimated 40 miles of riverine habitat. This habitat includes the Presumpscot River up to Mallison Falls Dam, and most of the Little River watershed, which is fed by aquifers in Buxton and Standish. However, connectivity between the main stem and two tributary streams – Inkhorn Brook and Colley Wright Brook – was severed following removal of the dams at Saccarappa Falls, when water levels in the former impoundment subsequently dropped by several feet, leaving the culverts perched above the Presumpscot River (photo, Fig. 3).

Being able to move freely between different types of habitats is vital for many aquatic species, and requires habitats that are well-connected by flowing waters. Connectivity is essential for anadromous fish, which are those species that are born in fresh water, migrate to the ocean for part of their lives, and migrate back to natal fresh water bodies to reproduce. Locally abundant anadromous species include alewives, blueback herring, shad, lamprey, and rainbow smelt. Eastern brook trout and other fish that require cold, oxygen rich water move seasonally among spring-fed streams, mainstem rivers, and estuaries to reproduce and feed. Connectivity is also essential for catadromous species such as American eel, which spends much of its adult life in freshwater and migrates to the ocean to reproduce. Terrestrial animals often travel and feed along rivers and streams.

Restoring Aquatic Connectivity a Key Aspect of Casco Bay Plan

The Casco Bay Plan prioritizes restoration of aquatic habitat connectivity, particularly to restore access for anadromous fish into historic spawning habitat. Construction of dams, roads, and railways in Casco Bay’s watershed severed many of these vital linkages and caused severe, and in some cases total, declines in anadromous fish populations. In addition to blocking movement of aquatic organisms, dams block passage between upstream and downstream habitats, and interrupt the natural movement of sediment, wood, nutrients, and floodwaters, while creating thermal sinks. Where roads and railways cross waterways, inadequate culverts often act like dams in their impact on riverine processes, including aquatic organism passage. Cold water species cannot thrive in the warm, oxygen impaired waters behind dams and perched culverts.

Figure 1. Timeline of dam removal projects since establishment of the Casco Bay National Estuary Program.

Figure 2. Stroudwater Dam, which is owned by the City of Portland and located at head-of-tide, cuts the Fore River Estuary off from over 31 miles of river and stream habitat in the Stroudwater River. The presence of river herring and other anadromous fish have been described in historic accounts of this river, the third largest water body that empties to Casco Bay. (Photo Matt Craig, CBEP.)

Figure 3. Following removal of the dams at Saccarappa Falls, upstream water levels dropped by several feet, creating a barrier for aquatic organisms trying to swim into Colley Wright Brook. A similar situation is present at Inkhorn Brook. Both structures lie below the Mountain Division Trail. (Photo Matt Craig, CBEP – 2021)

Figure 4. East Elm Street Dam, Yarmouth. Removal of this dam on the main stem of the Royal River would connect 61 miles of upstream river and stream habitat with the area currently accessible to anadromous fish following removal of Bridge Street Dam. (Photo Jerry Monkman – 2022)

Anadromous Fish Numbers Continue to Trend Upward

Figure 5. Plot of estimated blueback herring and alewife returns into the Presumpscot River watershed based on count data at Highland Lake Dam and Cumberland Mills Dam. Count data at Cumberland Mills were not provided to CBEP for the period 2023-2026, so returns are presumed to be higher than shown.

Anadromous fish monitoring is occurring at several sites in the Presumpscot watershed, including at Highland Lake Dam, Cumberland Mills Dam, and the Saccarappa fishways. Despite gaps in annual count data, annual returns of anadromous fish continue to trend upward.

Successful restoration of the anadromous fish runs into the Presumpscot River has generated enthusiasm and momentum for restoration of adjacent watersheds. On the Royal River, the Yarmouth Town Council voted to remove two dams near the head of tide. The first, Bridge Street Dam, was removed in August 2026. Addressing the next barrier upstream – East Elm Street Dam (Figure 4, above) – would connect an additional 61 miles of riverine habitat in the Royal River with Casco Bay.

Burgress Brook dam removal in Jugtown Forest

Multi-Group Efforts to Restore Aquatic Connectivity

Under leadership of Lakes Environmental Association and in close partnership with Trout Unlimited and Maine IFW, Sebago Clean Waters has focused on restoring connectivity between aquatic habitats within the Sebago Lake watershed, particularly on the Crooked River and tributary streams. Two dams, including Edes Falls Dam, were removed in recent years, while 12 Stream Smart culverts were installed. In addition to restoration of natural riverine processes, and improvements to our road network, this work benefits native fish such as Eastern brook trout and landlocked salmon, which spawn in coldwater rivers and streams, and other aquatic organisms.

LOOKING AHEAD TO 2030

Following removal of Bridge Street Dam on the Royal River in August of 2026, plans are in place to remove the next dam upstream, East Elm Street Dam, in 2027. This initiative to reconnect the Royal River and its tributaries with Casco Bay has created opportunities to study the effects of river restoration on water quality and ecological communities.. Several partners including Friends of Casco Bay, Gulf of Maine Research Institute, Royal River Alliance, Maine Department of Environmental Protection, and CBEP are monitoring the Royal River’s water quality, fish communities, and fish populations, providing information on how dam removal can change a river and estuary. Results will help inform evolving conversations about the future of dams elsewhere in the watershed and state.