Forgotten Legacies: Understanding and Mitigating Historical Human Alterations of River Corridors

Abstract

Legacies are persistent changes in natural systems resulting from human activities. Legacies that affect river ecosystems can result from human alterations outside of the river corridor, such as timber harvest or urbanization, or from alterations within the river corridor, including flow regulation, river engineering, and removal of large wood and beaver dams. Human alterations of river ecosystems have been occurring for thousands of years in some parts of the world and are now ubiquitous, yet both river scientists and the public may be unaware of the persistent effects of historical activities. Failure to recognize the legacy of historical activities that no longer occur can skew perceptions of river process and form and the natural range of variability in river ecosystems. Examples come from rivers of the Mid‐Atlantic Piedmont and the Pacific Northwest regions of the United States. Mid‐Atlantic Piedmont streams in which legacy sediment accumulated behind now‐abandoned mill dams experienced a complete transformation from wide, shallow, marshy valleys to sinuous rivers lined with tall cutbanks, but the existence and the cause of this river metamorphosis was not widely recognized until the first decade of the 21st century. Rivers of the Pacific Northwest from which large wood was removed have changed during the past century from spatially heterogeneous, multichannel systems closely connected to their floodplains via frequent channel avulsion and lateral migration to single‐thread channels with more homogeneous floodplains and less lateral connectivity. Again, this river metamorphosis has only been recognized within the past two decades. In each of these regional examples, river process and form have changed so substantially that the river ecosystems can be described as having assumed an alternative state. In these and many other examples, the alternative state provides lower levels of ecosystem services such as habitat, biodiversity, and attenuation of downstream fluxes of water, sediment, organic carbon, and nutrients. River management designed to enhance and restore these ecosystem services can be more effective if the continuing effects of these historical legacies are recognized. The grand scientific challenges resulting from historical human alterations of river ecosystems include the following: (1) to recognize the existence of a legacy that continues to affect river ecosystem process and form; (2) to understand the source of the legacy with respect to chronology, type, spatial extent, and intensity of human activities; (3) to understand the implications of the legacy regarding how river process and form and river ecosystem services have changed; and (4) to design management or restoration strategies that can mitigate the loss of river ecosystem services. In summary, the existence of forgotten legacies challenges river scientists to recognize the continuing effects of human activities that have long since ceased and also poses challenges for the application of scientific understanding to resource ma...

Coastal sedimentation across North America doubled in the 20th century despite river dams

“…The transport of these additional fine-grained sediment sources to coastal depocenters is highly non-linear in time and space 44 and may involve lag times from decades to centuries and net transport velocities of <10 m yr −1 45 . Pulses in fine-grained sediment transport 46 and ephemeral discharges from tributaries 47 have also been observed in response to increased precipitation in the lower watershed and the mobilization of legacy sediments 48 .…”

Biomechanical Properties and Resistance to Uprooting of Laboratory‐Scale Wood Logs

“…Within riparian zones, the hydrological, geomorphic, and ecological processes interact over wide spatial and temporal scales and contribute to bidirectional exchanges of energy and material (Bendix & Hupp, 2000; Johnston & Naiman, 1987; Hungr et al., 2001; Likens & Bormann, 1974; Pinay et al., 2018; Steiger et al., 2005). A key material exchange between rivers and adjacent riparian areas involves the transfer of wood logs to stream channels (Gurnell et al., 2005; Latterell et al., 2006; Naiman et al., 2000; Wohl, 2019), a process that often takes place after high magnitude flooding events (Comiti et al., 2016; Mao et al., 2013; Ruiz‐Villanueva, Piégay, et al., 2016; Zischg et al., 2018). The presence of wood material has been recognized to be as fundamental a component of woodland fluvial ecosystems as sediment and riparian vegetation (Abbe & Montgomery, 1996; Anderson et al., 1978; Beckman & Wohl, 2014; Gregory et al., 2003; A. Gurnell et al., 2002; Seo & Nakamura, 2009; Tockner et al., 2003; Wohl & Scott, 2017).…”

“…Transport refers to the mobilization of wood logs in river corridors. This is also influenced by river morphology, first‐order control on the wood regime (Wohl, 2019), and wood properties (e.g., orientation, size, and density) (Braudrick & Grant, 2000; A. Gurnell et al., 2002; Ruiz‐Villanueva, Wyżga, et al., 2016; Wohl, 2011). Finally, deposition is the process by which wood logs settle on floodplains and alluvial bedforms, such as bars and islands, as a result of low flow conditions or narrowing of the river section (Gasser et al., 2019).…”

Floods and rivers: a circular causality perspective

“…1). One should consider that channel geometry adjusts to climate 34,90,91 or anthropogenic pressure 15,29,92 , and according to channel-maintaining 93 and channel-changing discharges 26,62,94,95 . This form of feedback means that channel geomorphic response could cause a legacy of altered flood risk, that might be comparable to extreme events that might occur in the future.…”