**1. Introduction**

Humans alter geomorphology on an ever-increasing scale [1], one comparable with [2], and in some ways exceeding [3], rates of natural processes. Every change people make to the Earth's surface has the potential to affect the flow and accumulation of water. People have dug ditches, impounded streams and rivers, and otherwise shifted Earth's surface to direct and store water for human use, especially agriculture, for over 5000 years [1]. Today, land use matrices that entail human-engineered waterbodies, such as urban settlements, rice villages, and irrigated cropland, cover significant fractions of terrestrial Earth [4]. Patterns of surface water modification and extent are tightly linked to these land uses [5]. Man-made and -modified aquatic systems have become ubiquitous landscape features [6].

In spite of their commonness, artificial aquatic systems remain poorly understood. Indeed, it is often unclear which waterbodies even belong in the category of "artificial" or "anthropogenic". To date, the limited study of different artificial aquatic systems has been fragmented among various domains of ecology and other environmental sciences, and more often discussed on the margins of natural ecosystems than in conjunction with them, as part of a complete hydroscape [7]. Because science and managemen<sup>t</sup> does not often focus on artificial aquatic ecosystems, their abundance and extent are poorly quantified, and their ecological statuses and causes thereof poorly understood. As a result, we lack a scientific basis for assessing the ecological value of artificial aquatic systems, or determining how managemen<sup>t</sup> and policy might improve that value. The ubiquity of artificial aquatic systems, the potential commonalities among them and with natural aquatic ecosystems, and our limited understanding of their central and evolving role in the modern hydroscape all argue for more integrated study of the waterbodies created and transformed by human activity.

In this paper, we propose a framework for aquatic ecosystem artificiality that includes both the intent and magnitude of modification, and argue that policy and managemen<sup>t</sup> often implicitly use these characteristics to differentiate in their treatment of aquatic systems. We assemble current estimates of the extent of some types of artificial waterbodies in the U.S., and review established knowledge of their ecological condition, including the ecosystem services and disservices that they provide. We argue that the condition of artificial aquatic systems, as for their natural counterparts, likely reflects ecological processes and human decisions both in place and within their watersheds, and that the often poor condition of these systems [8] is not necessarily inherent to their anthropogenic origin. Finally, we posit that scientific undervaluation of artificial aquatic systems may lead scientists, managers, and policymakers to treat artificial waterbodies in ways that perpetuate poor ecological conditions. To manage the rapidly changing and increasingly anthropogenic hydroscape, aquatic scientists need to better inventory its myriad artificial components, evaluate their current structure and function, and link these findings to drivers. Future assessment of artificial aquatic systems that clarifies what their real and perceived values are, and how to purposefully change those values, will require more deliberate, intensive, systematic and mechanistic study, and, perhaps, a shift in our perspective regarding what counts as nature.
