Does the past haunt us? Landuse legacy and its consequences for hydrology and water quality

Much of earth's surface has an anthropogenic imprint. Humans have profoundly altered the environment and landscapes worldwide with changes so universal and intensive, that some have suggested, arguably, that humans have surpassed climate as a dominant force shaping landscapes. Examples of such ubiquitous modifications include: increasing greenhouse gases in the atmosphere; widespread land clearing for agriculture and mining with associated soil erosion; enrichment of soils, waters, and aquifers with nutrients such as nitrogen and phosphorus, and wholesale modifications and extinctions of plant and animal communities worldwide. There is increasing recognition that these legacies not only are substantially altering the current structure and function of landscapes and ecosystems worldwide, but also that the "memory" of these effects could persist for decades to millennia and may continue to affect earth surface processes far into the future. Understanding human legacies and their effects is thus central to advancing our knowledge of landscapes and ecosystems now and into the future.

Here, we highlight an example of one such legacy - colonial and post-colonial mill-damming in the eastern United States and elsewhere and accompanying accumulation of legacy sediments in valley-bottoms. While most of the milldams are breached, we are left with: (a) incised streams and eroding streambanks and sediment terraces; (b) streams that are hydrologically and biogeochemically disconnected from their floodplains and riparian zones; (c) bank erosion that, in some instances, exceeds upland sediment loads; (d) stream bottoms and hyporheic zones covered/clogged with legacy sediments; and (e) fine legacy sediments that contribute to nutrient loadings and processing in aquatic ecosystems. These "altered" conditions have major consequences for our understanding of watershed hydrology. Importantly, legacy sediments and their lag effects are not addressed in current watershed modeling, management, and regulatory (e.g., TMDL) assessments which are primarily based on contemporary upland loadings. Addressing these legacies will require novel approaches and paradigm shifts in science, management and policy decisions.