A longer vernal window: The role of winter coldness and snowpack in driving spring thresholds and lags

Climate change is altering the timing and duration of the vernal window, a period that marks the end of winter and the start of the growing season when rapid transitions in ecosystem energy, water, nutrient, and carbon dynamics take place. Research on this period typically captures only a portion of the ecosystem in transition and focuses largely on the dates by which the system wakes up. Previous work has not addressed lags between transitions that represent delays in energy, water, nutrient, and carbon flows. The objectives of this study were to establish the sequence of physical and biogeochemical transitions and lags during the vernal window period and to understand how climate change may alter them. We synthesized observations from a statewide sensor network in New Hampshire, USA, that concurrently monitored climate, snow, soils, and streams over a three-year period and supplemented these observations with climate reanalysis data, snow data assimilation model output, and satellite spectral data. We found that some of the transitions that occurred within the vernal window were sequential, with air temperatures warming prior to snow melt, which preceded forest canopy closure. Other transitions were simultaneous with one another and had zero-length lags, such as snowpack disappearance, rapid soil warming, and peak stream discharge. We modeled lags as a function of both winter coldness and snow depth, both of which are expected to decline with climate change. Warmer winters with less snow resulted in longer lags and a more protracted vernal window. This lengthening of individual lags and of the entire vernal window carries important consequences for the thermodynamics and biogeochemistry of ecosystems, both during the winter-to-spring transition and throughout the rest of the year

The Geomorphic and Hydrologic Impact of Beaver Dams on Headwater Streams in Northeastern Connecticut and Implications for River Restoration

The billion-dollar river restoration industry relies primarily on the concept of a free-flowing river to set restoration targets. However, rivers include natural barriers such as beaver dams, wood jams, glacial deposits, and bedrock constrictions. Following European colonization, most of these barriers were removed; some were replaced with far more homogenous ones such as human dams and road crossings. Although the biota intended to benefit from restoration evolved in rivers with natural barriers in place, little is known about the functions of the barriers that have been lost. Beaver dams—the subject of this dissertation—are just one type of the many natural barriers that should be considered by river restoration efforts. Chapter 2 presents a conceptual model of a river network that includes barriers, generating the fundamental hypothesis that intact and failed barriers create patchy features that store and release water and sediment. In chapter 3, a detailed geomorphic comparison of free-flowing and impounded channels shows that beaver dams decouple stream reaches, where distinct differences in adjacent channel reaches are explained by the presence of beaver dams. Observations of fine sediment deposits in steep reaches downstream of dams and of net sediment losses from old ponds support the hypothesis that beaver ponds store and release sediment. The hydrologic study of chapter 4 shows that the river channel through a beaver meadow loses water during rain events and subsequently gains water during recession, confirming the hypothesis of storage and release of water. Additional water gains during storm recessions in excess of the volume lost during the events, along with significantly lower runoff rates in the meadow channel during the events, suggest additional storage and subsequent release of upland runoff by the meadow. Chapter 5 examines summer water temperature at the streambed, which further demonstrates the patchiness generated by the intact and failed beaver dams. A distinct cold pool exists at a scour hole generated by a dam failure, and beaver dams buffer water temperatures upstream. As chapter 6 concludes, this patchiness should be further researched as a target for river restoration efforts where natural dams cannot be directly restored

Classification of the alterations of beaver dams to headwater streams in northeastern Connecticut, U.S.A.

Of the many types of barriers to water flow, beaver dams are among the smallest, typically lasting less than a decade and rarely exceeding 1.5 m in height. They are also among the most frequent and common obstructions in rivers, with a density often exceeding ten dams per km, a frequency of construction within a given network on a time scale of years, and a historic extent covering most of North America. Past quantification of the geomorphologic impact of beaver dams has primarily been limited to local impacts within individual impoundments and is of limited geographic scope. To assess the impact of beaver dams at larger scales, this study examines channel shape and sediment distribution in thirty river reaches in northeastern Connecticut, U.S.A. The study reaches fall within the broader categories of impounded and free-flowing segments, leaving a third segment class of beaver meadows requiring additional study. Each of the study reaches were classified at the reach scale as free-flowing, valley-wide beaver pond, in-channel beaver pond, and downstream of beaver dam. The bankfull channel width to depth ratios and channel widths normalized by watershed area vary significantly across the study reach classes. Additionally, reaches modified by beaver dams have finer sediment distributions. This paper provides the first quantitative geomorphic descriptions of the in-channel beaver pond and reaches downstream of beaver dams. Given the different channel shapes and sediment distributions, we infer that geomorphic processes are longitudinally decoupled by these frequent barriers that control local base level. These barriers generate heterogeneity within a river network by greatly increasing the range of channel morphology and by generating patches controlled by different processes. Therefore, in spite of the small size of individual beaver dams, the cumulative effect of multiple dams has the potential to modify processes at larger spatial scales. To improve assessment of the larger-scale impacts, we propose a hierarchical classification scheme based on discontinuities, place the reach classes of this study within that scheme, and suggest that further research should continue investigation of discontinuity at the network scale and quantification of the cumulative impacts

Introduction to the special issue on discontinuity of fluvial systems

Fluvial systems include natural and human-created barriers that modify local base level; as such, these discontinuities alter the longitudinal flux of water and sediment by storing, releasing, or changing the flow path of those materials. Even in the absence of distinct barriers, fluvial systems are typically discontinuous and patchy. The size of fluvial discontinuities ranges across scales from 100 m, such as riffles, to 104 m, such as lava dams or major landslides. The frequency of occurrence appears to be inversely related to size, with creation and failure of the small features, such as beaver dams, occurring on a time scale of 100 to 101 years and a frequency of occurrence at scales as low as 101 m. In contrast, larger scale discontinuities, such as lava dams, can last for time scales up to 105 years and have a frequency of occurrence of approximately 104 m. The heterogeneity generated by features is an essential part of river networks and should be considered as part of river management. Therefore, we suggest that “natural” dams are a useful analog for human dams when evaluating options for river restoration. This collection of papers on the studies of natural dams includes bedrock barriers, log jams and beaver dams. The collection also addresses the discontinuity generated by a floodplain — in the absence of an obvious barrier in the channel — and tools for evaluation of riverbed heterogeneity. It is completed with a study of impact of human dams on floodplain sedimentation. These papers will help geomorphologists and river managers understand the factors that control river heterogeneity across scales and around the world

Introduction to the special issue on discontinuity of fluvial systems

Fluvial systems include natural and human-created barriers that modify local base level; as such, these discontinuities alter the longitudinal flux of water and sediment by storing, releasing, or changing the flow path of those materials. Even in the absence of distinct barriers, fluvial systems are typically discontinuous and patchy. The size of fluvial discontinuities ranges across scales from 100 m, such as riffles, to 104 m, such as lava dams or major landslides. The frequency of occurrence appears to be inversely related to size, with creation and failure of the small features, such as beaver dams, occurring on a time scale of 100 to 101 years and a frequency of occurrence at scales as low as 101 m. In contrast, larger scale discontinuities, such as lava dams, can last for time scales up to 105 years and have a frequency of occurrence of approximately 104 m. The heterogeneity generated by features is an essential part of river networks and should be considered as part of river management. Therefore, we suggest that “natural” dams are a useful analog for human dams when evaluating options for river restoration. This collection of papers on the studies of natural dams includes bedrock barriers, log jams and beaver dams. The collection also addresses the discontinuity generated by a floodplain — in the absence of an obvious barrier in the channel — and tools for evaluation of riverbed heterogeneity. It is completed with a study of impact of human dams on floodplain sedimentation. These papers will help geomorphologists and river managers understand the factors that control river heterogeneity across scales and around the world

The River Discontinuum: Applying Beaver Modifications to Baseline Conditions for Restoration of Forested Headwaters

Billions of dollars are being spent in the United States to restore rivers to a desired, yet often unknown, reference condition. In lieu of a known reference, practitioners typically assume the paradigm of a connected watercourse. Geological and ecological processes, however, create patchy and discontinuous fluvial systems. One of these processes, dam building by North American beavers (Castor canadensis), generated discontinuities throughout precolonial river systems of northern North America. Under modern conditions, beaver dams create dynamic sequences of ponds and wet meadows among free-flowing segments. One beaver impoundment alone can exceed 1000 meters along the river, flood the valley laterally, and fundamentally alter biogeochemical cycles and ecological structures. In this article, we use hierarchical patch dynamics to investigate beaver-mediated discontinuity across spatial and temporal scales. We then use this conceptual model to generate testable hypotheses addressing channel geomorphology, natural flow regime, water quality, and biota, given the importance of these factors in river restoration