Legacy sediment (LS) refers to depositional bodies of sediment inherited from the increase of human activities since the Neolithic. These include a broad range of land use and land cover changes, such as agricultural clearance, lumbering and clearance of native vegetation, mining, road building, and urbanization, as well as alterations brought to river systems in the form of dams and other engineering structures meant to control and regulate natural fluvial processes (erosion, deposition, lateral migration, meandering). The concept of LS is used in geomorphology, ecology, as well as in water quality and toxicological studies. LS is distributed in spatially heterogeneous ways throughout a landscape and accumulates to form various landforms. It can progress through the fluvial system through facies changes from hillslope colluvium, to floodplain and wetland alluvium, to fine-grained lacustrine and estuarine slackwater deposits. The temporal nature of LS is time-transgressive, meaning that initiation and peak rates of deposition can take place at different times within a fluvial system, as well as at different times between regions. The intermittent transport of LS can be thought of as a cascading system that reworks LS deposits from hillslopes, into channels and onto floodplains, such that anthropogenic sediment will be mixed with and non-anthropogenic sediment. River systems record past and present imprints of anthropogenically-forced changes to the environment. LS is an element of change in this context, as it drives fluxes of energy and matter (connectivity) through fluvial systems and provides indication of past land-uses and river dynamics that can inform future trajectories of river response. In this sense, acknowledging the concept of LS can benefit informed policy development in stream restoration, water quality and sediment budget management, protection of aquatic ecosystems, and flood risk. Moreover, the implications of legacy effects associated with anthropogenically modified sediment dynamics are critical in the context of ecosystem services.
Definitions
Post-settlement alluvium Definitions predominantly indicate post-settlement alluvium North America created as a result of agricultural clearance. “Legacy Sediment (n.) Are sediments that (1) were eroded from upland slopes during several centuries of intensive land clearing, agriculture, and milling (in the eastern U.S., this occurred from the late 17th to late 19th Centuries); (2) collected along stream corridors and valley bottoms, burying pre-settlement streams, floodplains, wetlands, and dry valleys; and that altered the hydrologic, biologic, aquatic, riparian, and chemical functions of pre-settlement streams and floodplains; (3) accumulated behind ubiquitous low-head mill dams in slackwater environments, resulting in thick accumulations of fine-grained sediment, which distinguishes ‘‘legacy sediment’’ from fluvial deposits associated with meandering streams; (4) can also accumulate as coarser grained, more poorly sorted colluvial (not associated with stream transport) deposits, usually at valley margins; (5) can contain varying amounts of total phosphorus and nitrogen, which contribute to nutrient loads in downstream waterways from bank erosion processes. . .’’
Anthropogenically-caused episodic sedimentary deposits As a result of criticism related to the limited scope and applicability of this definition, a more flexible and generic definition has been proposed that (1) includes a broader range of human activities, (2) considers more sediment types apart from post-settlement alluvium, and (3) respects the spatial (nonuniform) and temporal (time-transgressive) variability of LS: "Legacy sediment: Earth materials—primarily alluvium [or colluvium]—deposited following human disturbances such as deforestation, agricultural land use, or mining. The phrase is often used to describe post-European floodplain sediments, also known as post settlement alluvium. Awareness of legacy sediment has grown in response to the importance it plays in sediment budgets, water quality, river restoration, toxicity, lateral channel connectivity, and geomorphic theory. . .’’ "Legacy sediment is primarily alluvium [and colluvium] that was deposited following human disturbances in a watershed. The disturbance may have been in the form of deforestation, plowing agricultural land, mining, or other land-use changes. In North America and Australia, legacy sediments are ubiquitous and represent episodic erosion in response to the colonization of land by European settlers who introduced Old World land- clearance technologies (e.g. steel tools and plows pulled by draft animals) and export economies. In these settings, legacy sediments are often described as post-settlement alluvium (PSA), which may cover entire floodplains and bury the pre- settlement soil with a thick mantle of relatively young stratified sediment."
Types and related landforms
Types LS encompasses sediment of differing structures and textures. They can be colluvial, containing poorly sorted, angular rock fragments deposited by mass wasting or sheet erosional processes, alluvial, containing well sorted, rounded clasts and very-fine grained suspended sediment deposited by fluvial processes.
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