Anthropocene streams and base-level controls from historic dams in the unglaciated mid-Atlantic region, USA

Abstract

Recently, widespread valley-bottom damming for water power was identified as a primary control on valley sedimentation in the mid-Atlantic US during the late seventeenth to early twentieth century. The timing of damming coincided with that of accelerated upland erosion during post-European settlement land-use change. In this paper, we examine the impact of local drops in base level on incision into historic reservoir sediment as thousands of ageing dams breach. Analysis of lidar and field data indicates that historic milldam building led to local base-level rises of 2–5 m (typical milldam height) and reduced valley slopes by half. Subsequent base-level fall with dam breaching led to an approximate doubling in slope, a significant base-level forcing. Case studies in forested, rural as well as agricultural and urban areas demonstrate that a breached dam can lead to stream incision, bank erosion and increased loads of suspended sediment, even with no change in land use. After dam breaching, key predictors of stream bank erosion include number of years since dam breach, proximity to a dam and dam height. One implication of this work is that conceptual models linking channel condition and sediment yield exclusively with modern upland land use are incomplete for valleys impacted by milldams. With no equivalent in the Holocene or late Pleistocene sedimentary record, modern incised stream-channel forms in the mid-Atlantic region represent a transient response to both base-level forcing and major changes in land use beginning centuries ago. Similar channel forms might also exist in other locales where historic milling was prevalent.

The Anthropocene: a new epoch of geological time?

“…Merritts et al [20] show that conceptual models linking channel condition and sediment yield exclusively with modern upland land use are incomplete for valleys impacted by mill dams. With no equivalent in the Holocene or Late Pleistocene sedimentary record, modern incised stream channel forms in the mid-Atlantic region of the USA represent a transient response to both base-level forcing and major changes in land use beginning centuries ago.…”

Developing predictive insight into changing water systems: use-inspired hydrologic science for the Anthropocene

“…Hydrologists have already attempted empirical reconstructions of hydrological change and water system histories in specific locations, generally motivated by the need to understand the genesis of contemporary hydrological problems (An Heyvaert and Baeteman, 2008;Merritts et al, 2011;Nicholson, 1979;Schuldenrein et al, 2004). While empirically informative, these studies could also provide valuable theoretical test beds for co-evolutionary models over decadal-to century-scale time frames.…”

“…The value of "typical" paleo-hydrological proxies (Baker et al, 1993;Jarrett, 1991) for evaluating century-scale changes in watersheds is not well understood. Accessing the hydrologically relevant information embedded in historical data sets -for instance information regarding construction of run-of-river dams for mill operation in the USA (Merritts et al, 2011;Walter and Merritts, 2008) -is likely to require interdisciplinary research, methods for fusing "hard" and "soft" data, and ways to address the reliability of information derived from "human sensors" (Hall et al, 2010).…”

Nitrate retention capacity of milldam-impacted legacy sediments and relict A horizon soils

“…The deposition of fine-grained legacy sediment throughout the mid-Atlantic region, in particular, has led to the burial of once biogeochemically active riparian valley bottoms (Merritts et al, 2005Walter and Merritts, 2008a), which, in turn, has altered nutrient cycling dynamics at the land-stream-water interface (Meade et al, 1990;Renwick et al, 2005;Walter et al, 2007;Walter and Merritts, 2008a;Merritts et al, 2011;Weitzman et al, 2014). Legacy sediments introduce two key problems for water quality.…”

“…Similarly, serving as a case study for the research of this study, the historic, post-European settlement of the midAtlantic region in the US in the seventeenth and eighteenth centuries was characterized by rapid anthropogenic landscape modifications, consisting of intense land clearing, deforestation, and the construction of tens of thousands of milldams (Walter and Merritts, 2008a;Merritts et al, 2011). Pervasive land clearing led to increased sedimentation rates throughout the Chesapeake Bay watershed (Jacobson and Coleman, 1986;Brush, 2009), with much of this sediment being deposited and stored behind small (∼ 2.5-3.7 m high), valley-spanning milldams (Walter and Merritts, 2008a).…”