Geology topics

Research about Penobscot River

Source-linked reports with geographic coverage including Penobscot River.

At least 19 recordsLinked to original sources

USGS2025

Seaward movements and mortality of Atlantic salmon Salmo salar L. kelts in the Penobscot River, Maine

The Atlantic salmon Salmo salar L. is an endangered species in the United States, but post-spawn downstream movements remain poorly understood. We conducted a 2-year acoustic telemetry study to characterize downstream movements and to quantify apparent mortality and downstream passage of post-spawn adults (kelts) in the Penobscot River, Maine, USA ( N = 112). The majority of tagged S. salar kelts (54%) exhibited a rapid movement seaward post-release instead of overwintering in the river. Salmo salar kelts that overwintered exhibited two patterns, distinguished by whether a dam was present or not. We found no relationship between post-release movement pattern, sex, body condition index or release year. Estimated apparent mortality rates were high (83%), with a greater probability of mortality occurring near the dams and release site. Apparent mortality rate did not differ by sex, body condition or year, but it was greater for individuals that moved out directly post-release and less for those that overwintered. While our results may suggest that overwintering in the river is a favourable pattern for survival, these results may be related to the closures of the designated downstream passages at the first two dams encountered by S. salar kelts. In fact, 55% of S. salar kelts passed a dam when the designated passages were closed. Altogether, these results may demonstrate the complex nature of downstream passage for S. salar kelts, particularly when having to navigate multiple dams.

Penobscot River↗

USGS2025

The story of the Penobscot River Ecology Mural: A 10-step process for scientists to create public art

Rivers are home to a wide variety of biota, including fishes, amphibians, reptiles, birds, freshwater mussels, aquatic insects, and microscopic organisms that fill unique niches to support broader ecosystem functions. While the general public may be aware of recreationally relevant biological life (e.g., fishes and insects to model flyfishing flies from), they may not be aware of more obscure yet important taxa. For example, freshwater mussels (Unionidae) improve water quality and clarity of rivers through filtration ( Vaughn, 2018), which is vital to riverine ecosystems. Despite this importance, lack of public knowledge about freshwater mussels is the second biggest challenge to their management ( Bouska et al., 2018). How do we best get the public to see value in underappreciated taxa to support their conservation? Public art installations can spread awareness about science effectively because they distill information for broader audiences, are eye-catching, and reach people through an emotional, aesthetic, and personal lens ( Thompson et al., 2023). While invertebrates are generally viewed by the public with fear and aversion ( Kellert, 1993) this can be ameliorated through interaction and learning, particularly for children ( Miller et al., 2025). Through cross-disciplinary collaboration between artists, scientists and the local community, we created the “Penobscot River Ecology Mural,” a scientifically informed artwork featuring underappreciated taxa like mussels and insects. In this article, we will elucidate the story of how we successfully made this piece through 10 steps, which provide a blueprint for future endeavors.

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USGS2024

Peering into the eye of the sea lamprey: What can stable isotopes in lamprey eye lenses reveal about their life history?

The Atlantic sea lamprey ( Petromyzon marinus ) is an anadromous species which is a detritivore for years in freshwater, before metamorphosing into a juvenile and feeding parasitically at sea. However, their migratory patterns and marine ecology are poorly characterized. In other fishes, chemical analysis of calcified structures has aided in understanding their life history, but sea lamprey lacks these structures. Recent work using eye lenses suggests an alternate approach. Lenses are stable protein structures that accrue layers on their outer edge as they grow. We used stable isotope ratios (δ x Y) of hydrogen, carbon, nitrogen, and oxygen to explore lens layers and compare lenses to muscle. Returning adults were collected in May 2021 from the Penobscot (Maine), Connecticut (Massachusetts), and Susquehanna (Maryland) rivers. Lenses were dissected into three layers (surface, middle, core) and analyzed separately to investigate changes over time. We used mixed-effects models to assess differences in isotope ratios among lens layers. Isotope ratios changed from the core to the surface layer and δ 13 C were suggestive of a change from terrestrial to marine sources and δ 15 N showed an increase in trophic level as individuals grew, as would be expected when transitioning from detrital sources to marine prey. However, the isotope ratios of the outer lens layer generally did not correspond with the muscle, perhaps indicating a different temporal interval. These data suggest temporal accretion of lens proteins provide a record to explore sea lamprey ecology.

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USGS2024

Dams facilitate predation during Atlantic salmon (Salmo salar) smolt migration

Diadromous fish populations have incurred precipitous declines across the globe. Among many stressors, these species are threatened by anthropogenic barriers that impede movement, alter riverine habitat, and augment predator communities. In this study, we used acoustic transmitters ( n = 220) with predation and temperature sensors to characterize Atlantic salmon ( Salmo salar ) smolt predation risk in the Penobscot River, Maine, USA. Across two seasons, we documented 79 predation events through a 170 km migratory pathway, which included three hydropower projects and a large estuary. We detected tagged smolts that were predated by fish ( n = 42), marine mammals ( n = 28), and birds ( n = 9). Using a multistate mark-recapture framework, we estimated that 46% of smolts were predated during downstream migration, which accounted for at least 55% of all mortality. Relative predation risk was greatest through impoundments and the lower estuary, where on average, predation rates were 4.8-fold and 9.0-fold greater than free-flowing reaches, respectively. These results suggest that predation pressure on Atlantic salmon smolts is exacerbated by hydropower projects and that predation in the lower estuary may be greater than expected.

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USGS2023

Seven dam challenges for migratory fish: Insights from the Penobscot River

More than a century of impoundments in the Penobscot River, Maine, USA, has contributed to population declines in migratory fish in the system. A decade of change, research, and monitoring has revealed direct and indirect ways that dams have influenced the river habitat, connectivity for migratory fish, and the food web. The removal of two main-stem dams (in 2012 and 2013) and bolstering of fish passage have been part of coordinated restoration efforts in the watershed. Integral to this undertaking was support for short- and long-term monitoring and research that included physical habitat, fish passage, and broad scale ecological assessments. Herein we discuss the seven interconnected and complex ways that dams have affected the Penobscot River ecosystem, particularly for migratory fish. These include familiar influences ascribed to dams: i) impaired access to habitat, ii) injury and mortality, and iii) delays of migration. Other ecological influences are less studied and more subtle: iv) facilitation of predation, v) community shifts, and vi) demographic shifts. Lastly, dams result in vii) a loss of ecosystem services that would otherwise be intact in an unimpounded system. We draw on both direct examples from the Penobscot River and broader information to characterize how impoundments have transformed this ecosystem for more than a century. Recent dam removals and mitigation efforts have reestablished some of these ecological functions.

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USGS2023

Fish assemblages in the Penobscot River: A decade after dam removal

The Penobscot River Restoration Project in Maine was a large river rehabilitation project that culminated in the removal of the two lowermost dams and improvements to fish passage on several remaining dams. Fish assemblages were surveyed for 3 years prior to rehabilitation, 3 years after rehabilitation, and 8 years after rehabilitation. Approximately 475 km of shoreline were sampled via boat electrofishing, yielding 133,394 individual fish of 41 species. The greatest shifts in assemblage structure occurred immediately after dam removal in formerly impounded sections, with an increased prevalence of riverine and migratory species. Long-term sampling documented changes within tributaries and tidally influenced river segments, where large schools of adult and young-of-the-year alosines increased in abundance. Upstream of the lowermost dam, the river remains dominated by lacustrine species, while adult anadromous fishes continue to be most abundant immediately downstream of the lowermost dam. Our results provide increased evidence that dam removals result in altered fish assemblages, which are now dominated by riverine and anadromous species in previously impounded habitats. Alosines in the Penobscot River have exhibited the greatest long-term response to river restoration efforts.

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USGS2022

Effects of dam-induced delays on system-wide survival of Atlantic salmon smolts during high-flow, high-survival years in the Penobscot River, Maine, USA

System-wide survival of hatchery-reared Atlantic salmon ( Salmo salar ) smolts was evaluated (2017–2019) in the Penobscot River and compared to survival estimates from previous years that spanned major changes (2005–2016). This system was transformed through two dam removals in 2012 and construction of a nature-like passage structure at a third. The main stem had three dams (five prior to 2012), while the main tributary had four dams (one with the new nature-like passage). We estimated survival using acoustic telemetry mark–recapture ( n = 1482) from 2017 to 2019. Six release sites and two release dates were included to assess system-wide survival. Survival from 2017 to 2019 was higher than previous years, with total cumulative survival > 0.75, independently of year and release sites, compared to survival < 0.5 in previous years. These years coincided with exceptional high flows not seen previously. We found an effect of delays on survival, longer delays associated with lower survival. Overall, survival in these years increased in all reaches relative to previous years except for one dam, Weldon Dam, which was a site of sustained high mortality.

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USGS2021

The consequences of dam passage for downstream-migrating American eel in the Penobscot River, Maine

American eel ( Anguilla rostrata ) often pass hydropower dams during adult spawning migrations. We conducted a 4-year acoustic telemetry study that characterized passage risks through two dams (West Enfield and Milford) in the Penobscot River, Maine, USA. We released tagged fish ( n = 355) at two sites, estimated survival and delay under variable river conditions, and compared performance among dammed and free-flowing river sections. Survival rates (standardized per river kilometre, rkm) were lower at West Enfield (Φ rkm = 0.984 ± 0.006 SE) and Milford (Φ rkm = 0.966 ± 0.007 SE) compared with undammed River sections (Φ rkm = 0.998 ± 0.0003 SE). Cumulative mortality was 8.7% (4.4 km) and 14.2% (5.5 km) through dammed sections and 8.7% throughout the rest of the river (58.1 km). Fish that already passed an upstream dam incurred higher downstream mortality compared with individuals without passage experience. Additionally, fish endured long delays at dams, and >10% of fish were delayed >24 h. Low flows exacerbated the risk of mortality and delay. These results offer evidence for direct, latent, and sublethal consequences of dam passage for migrating eels.

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USGS2020

River channel response to dam removals on the lower Penobscot River, Maine, United States

Most geomorphology studies of dam removals have focused on sites with appreciable quantities of stored sediments. There is great interest in channel responses to sediment releases because of potential effects on aquatic and riparian habitats and human uses of these areas. Yet, behind many dams in the Northeast U.S. and other regions of the world only minor accumulations of sediment are present because of small impoundments, run‐of‐river dam design and management (inflow ≈ outflow), low watershed sediment yield, and/or channel beds dominated by coarse sediment and/or bedrock. The two lowermost dams on the Penobscot River in Maine, United States, removed in 2012–2013, exemplified those conditions. Great Works and Veazie dams were about 6 and 10 m high, respectively. Pre‐project geophysical surveys showed coarse substrates dominated the reservoir beds and little sediment was stored in either impoundment—functions of reach geology, late Quaternary history, and upstream dams. Repeat cross‐section surveys in each impoundment, as well as the upstream and downstream reaches, were completed from 2009 to 2015 to evaluate channel morphology responses to the removals. Bed‐sediment grain size and turbidity were also measured to characterize changes in bed texture and suspended sediment. Pre‐ and post‐removal survey comparisons confirmed the expectation that bed elevations, channel shapes, and channel positions would not change substantially. Changes were often within, or close to, our estimated random measurement error. Our study shows that large‐scale physical changes are likely to be minimal when impoundments storing relatively little sediment are removed from erosion‐resistant streambeds. Many dams eligible for removal have these characteristics, making these observations an important case study that is largely unrepresented in the dam removal literature.

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USGS2020

Balancing fish-energy-cost tradeoffs through strategic basin-wide dam management

Dam management often involves tradeoffs among hydropower generation capacity, environmental impacts, and project costs. However, our understandings of such tradeoffs under a full range of dam management options remain limited, which hinders our ability to make sound and scientifically defensible dam management decisions. In order to assess the scope for theoretical tradeoffs, a dynamic model of hydropower production, important fish populations, and project costs was developed using the system dynamics modeling technique. Three dam management options investigated the likely outcomes from: dam removal, fishway installation (e.g., pool-and-weir, Denil, and fish lift), and no action. The model was applied to the Penobscot River located in Maine, United States as a proof of concept, where recent actions (i.e., dam removal and fishway construction) have been undertaken. We modeled theoretical influence of these actions on four significant sea-run fish (alewife Alosa pseudoharengus , American shad Alosa sapidissima , Atlantic salmon Salmo salar , and sea lamprey Petromyzon marinus ) by developing an index of spawner population potential based on population models for each species. Optimal dam management solutions may maximize spawner population potential and energy production to 60-62% of maximum achievable values while limiting the project cost to US$17 million (44% of the maximum value). Our results demonstrate that basin-scale management strategies may increase the migratory fish restoration while preserving hydropower generation capacity. Diversification of management options (e.g., combination of fishway installations, dam removals, and generation capacity) may increase the efficacy of strategic fish-energy-cost tradeoffs.

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USGS2019

Managing dams for energy and fish tradeoffs: What does a win-win solution take?

Management activities to restore endangered fish species, such as dam removals, fishway installations, and periodic turbine shutdowns, usually decrease hydropower generation capacities at dams. Quantitative analysis of the tradeoffs between energy production and fish population recovery related to dam decision-making is still lacking. In this study, an integrated hydropower generation and age-structured fish population model was developed using a system dynamics modeling method to assess basin-scale energy-fish tradeoffs under eight dam management scenarios. This model ran across 150 years on a daily time step, applied to five hydroelectric dams located in the main stem of the Penobscot River, Maine. We used alewife ( Alosa pseudoharengus ) to be representative of the local diadromous fish populations to link projected hydropower production with theoretical influences on migratory fish populations on the model river system. Our results show that while the five dams can produce around 427 GWh/year of energy, without fishway installations they would contribute to a 90% reduction in the alewife spawner abundance. The effectiveness of fishway installations is largely influenced by the size of reopened habitat areas and the actual passage rate of the fishways. Homing to natal habitat has an insignificant effect on the growth of the simulated spawner abundance. Operating turbine shutdowns during alewives' peak downstream migration periods, in addition to other dam management strategies, can effectively increase the spawner abundance by 480–550% while also preserving 65% of the hydropower generation capacity. These data demonstrate that in a river system where active hydropower dams operate, a combination of dam management strategies at the basin scale can best balance the tradeoff between energy production and the potential for migratory fish population recovery.

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USGS2019

A dam passage performance standard model for American shad

Objectives for recovery of alosines commonly involve improving fish passage at dams during migration. However, a quantitative basis for dam passage performance standards is largely absent. We describe development of a stochastic life-history-based simulation model for American shad, Alosa sapidissima , to estimate effects of dam passage and migratory delay on abundance, spatial distribution of spawning adults, and demographic structuring in space and time. We used the Penobscot River, Maine, USA, as a case study to examine sensitivity of modeled population metrics and probability of achieving specific management goals to inputs. Spawner abundance and percentage of repeat spawners were most sensitive to survival and migration delay at dams, marine survival, and temperature cues for migratory events. Recovery objectives related to abundance and spatial distribution of spawners were achievable under multiple scenarios, but high rates of upstream and downstream passage were necessary. The simulation indicated trade-offs between upstream and downstream passage efficacy whereby increased downstream passage was required to maintain or increase population abundance in conjunction with increased upstream passage. This model provides a quantitative support tool for managers to inform ecologically based decisions about a suite of management scenarios to facilitate recovery and sustainability of diadromous fish populations.

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USGS2019

Role of recovering river herring population on smallmouth bass diet and growth

Fish assemblages in Atlantic coastal rivers have undergone extensive ecological change in the last two and a half centuries due to human influence, including extirpation of many migratory fish species, such as river herring (Alosa spp.) and introduction of nonnative piscivores, notably Smallmouth Bass Micropterus dolomieu. Recently, dam removals and fish passage improvements in the Penobscot River, Maine, have allowed river herring to return to reaches of the river that have been inaccessible since the late 19th century. Alosine populations have increased and this trend is anticipated to continue. This may increase forage in the system which could potentially increase growth for Smallmouth Bass, the dominant piscivore. We examined the diet and growth of Smallmouth Bass collected from areas of the Penobscot River watershed with and without access to river herring as prey. We collected 765 Smallmouth Bass throughout 2015, examined the stomach contents of 573 individuals, and found notable differences in diet among three river reaches with common seasonal trends. Juvenile river herring composed an average of 19% (SE = ±6%) of stomach contents by mass from Smallmouth Bass collected in the freshwater tidal area but were rarely observed in the diets upstream. We used estimates from von Bertalanffy growth models to examine differences in growth among reaches and found that asymptotic length was the longest (425 mm TL) in the Tidal reach where access to river herring was unrestricted. We then used these data to predict changes to growth associated with increased access to juvenile river herring prey with bioenergetics models. Results indicated that substituting juvenile river herring for less energy-dense prey (e.g.,invertebrates) may lead to increases in seasonal growth throughout the watershed as river herring populations continue to rebound in response to dam removal. Our results provide insight into the diet and growth of Smallmouth Bass in a large New England river, and provide a foundation for future work investigating unfolding changes to these characteristics following recent dam removals.

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USGS2018

Dam Removal and Fish Passage Improvement Influence Fish Assemblages in the Penobscot River, Maine

Dams and their impoundments disrupt river habitat connectivity to the detriment of migratory fishes. Removal of dams improves riverine connectivity and lotic habitat, which benefits not only these fishes but also resident fluvial specialist species. Restoration efforts on the Penobscot River, Maine, are among the largest recently completed in the United States and include the removal of the two lowermost dams and improvements to fish passage at several remaining barriers. We assessed fish assemblages in the main‐stem river and several major tributaries before (2010–2012) and after (2014–2016) dam removal using boat electrofishing surveys and a stratified random sampling design. In total, we sampled 303 km of shoreline and captured 107,335 individual fish representing 39 species. Similarity indices and rarefaction curves indicated that significant changes in fish assemblage composition occurred in reaches that underwent both habitat and connectivity changes (i.e., directly above removed dams). The newly connected reaches became more similar in fish assemblage composition, as demonstrated by an average increase of 31% in similarity scores. The changes in similarity score in these reaches were driven by increasing access for anadromous fishes and decreasing abundances of slow‐water specialist species. For example, we observed a marked reduction in lacustrine species in former impoundments. These assemblage shifts were further illustrated by nonmetric multidimensional scaling in which sites directly above former dams exhibited the largest ordinal shifts immediately following dam removal. We also found all anadromous species in greatest abundance below the lowermost dam during each respective sampling period, though we did find some anadromous species above the lowermost dam during postremoval sampling. Our results demonstrate the potential for large dam removal projects to restore both fluvial and anadromous fish assemblages.

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USGS2017

Size selection from fishways and potential evolutionary responses in a threatened Atlantic salmon population

The evolutionary effects of harvest on wild fish populations have been documented around the world; however, sublethal selective pressures can also cause evolutionary changes in phenotypes. For migratory fishes, passage facilities may represent instances of nonlethal selective pressure. Our analysis of 6 years of passage data suggests that certain fish passage facilities on the Penobscot River have been exerting selective pressure against large-bodied, anadromous Atlantic salmon ( Salmo salar ). At the second and third dams in the river, a 91-cm salmon was 21%–27% and 12%–16% less likely to pass than a 45-cm salmon, respectively. Fish size positively influences egg survival and number and is a heritable trait. Therefore, in a wild-reproducing population, exclusion of large fish from spawning areas may have population-level impacts. In the Penobscot River, most returning adults derive from a hatchery program that collects its broodstock after passing the first dam in the river. Analysis of fork lengths of salmon returning to the Penobscot River from 1978 to 2012 provided mixed support for evolution of size at maturity in different age classes in a pattern that may be expected from interactions with conservation hatchery operations. Additionally, slow-maturing and iteroparous individuals that represent the largest salmon size classes were essentially lost from the population during that time, and Penobscot River fish have shorter fork lengths at maturity than Atlantic salmon in undammed systems.

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USGS2016

Upstream movements of Atlantic Salmon in the Lower Penobscot River, Maine following two dam removals and fish passage modifications

The Penobscot River Restoration Project (PRRP), to be completed in 2016, involved an extensive plan of dam removal, increases in hydroelectric capacity, and fish passage modifications to increase habitat access for diadromous species. As part of the PRRP, Great Works and Veazie dams were removed, making Milford Dam the first impediment to federally endangered Atlantic Salmon Salmo salar . Upstream habitat access for Atlantic Salmon is dependent upon successful and timely passage at Milford Dam because nearly all suitable spawning habitat is located upstream. In 2014 and 2015, a total of 73 adult salmon were radio-tagged to track their upstream movements through the Penobscot River to assess potential delays at (1) the dam remnants, (2) the confluence of the Stillwater Branch and the main stem of the Penobscot River below the impassable Orono Dam, and (3) the Milford Dam fish lift (installed in 2014). Movement rates through the dam remnants and the Stillwater confluence were comparable to open river reaches. Passage efficiency of the fish lift was high in both years (95% and 100%). However, fish experienced long delays at Milford Dam, with approximately one-third of fish taking more than a week to pass in each year, well below the Federal Energy Regulatory Commission passage standard of 95% within 48 h. Telemetry indicates most fish locate the fishway entrance within 5 h of arrival and were observed at the entrance at all hours of the day. These data indicate that overall transit times through the lower river were comparable to reported movement rates prior to changes to the Penobscot River due to the substantial delays seen at Milford Dam. The results of this study show that while adult Atlantic Salmon locate the new fish lift entrance quickly, passage of these fish was significantly delayed under 2014–2015 operations.

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USGS2015

Fish community response to dam removal in a Maine coastal river tributary

Sedgeunkedunk Stream, a third-order tributary to the Penobscot River in Maine, historically has supported several anadromous fishes including Atlantic Salmon Salmo salar , Alewife Alosa pseudoharengus , and Sea Lamprey Petromyzon marinus . Two small dams constructed in the 1800s reduced or eliminated spawning runs entirely. In 2009, efforts to restore marine–freshwater connectivity in the system culminated in removal of the lowermost dam (Mill Dam) providing access to 4.7 km of lotic habitat and unimpeded passage into the lentic habitat of Fields Pond. In anticipation of these barrier removals, we initiated a modified before-after-control-impact study, and monitored stream fish assemblages in fixed treatment and reference sites. Electrofishing surveys were conducted twice yearly since 2007. Results indicated that density, biomass, and diversity of the fish assemblage increased at all treatment sites upstream of the 2009 dam removal. No distinct changes in these metrics occurred at reference sites. We documented recolonization and successful reproduction of Atlantic Salmon, Alewife, and Sea Lamprey in previously inaccessible upstream reaches. These results clearly demonstrate that dam removal has enhanced the fish assemblage by providing an undisrupted stream gradient linking a small headwater lake and tributary with a large coastal river, its estuary, and the Atlantic Ocean.

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USGS2015

Initiation of migration and movement rates of Atlantic salmon smolts in fresh water

Timing of ocean entry is critical for marine survival of both hatchery and wild Atlantic salmon ( Salmo salar ) smolts. Management practices and barriers to migration such as dams may constrain timing of smolt migrations resulting in suboptimal performance at saltwater entry. We modeled influences of stocking location, smolt development, and environmental conditions on ( i ) initiation of migration by hatchery-reared smolts and ( ii ) movement rate of hatchery- and wild-reared Atlantic salmon smolts in the Penobscot River, Maine, USA, from 2005 through 2014 using acoustic telemetry data. We also compared movement rates in free-flowing reaches with rates in reaches with hydropower dams and head ponds. We compared movement rates before and after (1) removal of two mainstem dams and (2) construction of new powerhouses. Initiation of movement by hatchery fish was influenced by smolt development, stocking location, and environmental conditions. Smolts with the greatest gill Na + , K + -ATPase (NKA) activity initiated migration 24 h sooner than fish with the lowest gill NKA activity. Fish with the greatest cumulative thermal experience initiated migration 5 days earlier than those with lowest cumulative thermal experience. Smolts released furthest from the ocean initiated migration earlier than those released downstream, but movement rate increased by fivefold closer to the ocean, indicating behavioral trade-offs between initiation and movement rate. Dams had a strong effect on movement rate. Movement rate increased from 2.8 to 5.4 km·h −1 in reaches where dams were removed, but decreased from 2.1 to 0.1 km·h −1 in reaches where new powerhouses were constructed. Movement rate varied throughout the migratory period and was inversely related to temperature. Fish moved slower at extreme high or low discharge. Responses in fish movement rates to dam removal indicate the potential scope of recovery for these activities.

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