*Construction, Transformation, and Alteration*

The most clearly artificial aquatic systems are those that humans construct where none existed before. Even the existence of these constructed artificial ecosystems relates to other natural waterbodies because they represent water that could have gone or stayed elsewhere, on the landscape, underground, or to other parts of the hydrologic cycle. Deliberate examples of construction include fountains, many roadside ditches, rain gardens, stormwater treatment areas, many farm ponds, and all designer ecosystems [10]. Conservation-oriented water regulation typically exempts such constructs outright; they usually do not count as water [7,23,24]. Accidental examples include logging ruts, erosional gullies in building sites and agricultural fields, poorly drained impervious surfaces, and even bomb craters [25]. Some of these accidental features represent failures of water conservation regulation or other damaging abstraction from natural water sources. Nonetheless, left unmaintained, in time, such accidental waterbody construction in relative uplands can "naturalize" to a seemingly "wild" ecosystem [25–27]. Waterbodies that humans have constructed accidentally, but that appear relatively free from human intent, are more likely to be regulated than waterbodies that humans have constructed on purpose [23,24].

Transformation occurs when human intervention changes waterbodies from one type to another, fundamentally different in morphology and flow, such as from a lake to a wetland, or a wetland to a stream. Deliberate transformations include ditching of wetlands for agriculture, conversion of wetlands to ponds during development, damming of streams to build reservoirs, piping of creeks, and many restorations. The impoundment of the Everglades behind Tamiami Trail, a road that interrupted sheet flow, is one prominent such example [28]. Similarly, humans accidentally transformed the bed of the Salt River, dried through damming upstream, into wetlands, at stormwater outflows [15]. Transformed waters generally retain their regulatory status [23,24] regardless of intent. Such transformations are often considered degrading and thus may require regulatory permission [23,29,30].

*Water* **2018**, *10*, 1096

The least modification that might make a waterbody appear artificial is alteration, in which fundamental morphology and flow are retained. Deliberately, people straighten and channelize rivers, harden riverbanks and shorelines, and dredge lakes. Accidentally, sedimentation from agriculture, mining, or construction makes streams and lakes shallower [31]; mill dams clogged river valleys all over the eastern U.S. [32]. Meanwhile, "urban stream syndrome", including incision, flashiness, and other changes largely in response to stormwater drainage, has become a well-known issue in developed areas everywhere [33]. In one particularly grand example of accidental alteration, a railroad causeway divides the Great Salt Lake into mostly independent halves with very different chemistry and community assemblages [34]. Alterations generally do not remove jurisdiction of regulation from waterbodies, and instead likely invoke regulatory oversight [23,29,30]. Of the range of artificial aquatic systems, altered waterbodies are often the easiest to imagine in their "natural" state, the likeliest to be labeled as simply "degraded", the most likely to attract conservation, restoration, and related scientific interest, and, depending on context, possibly the most appropriate for restoration [35–37].

Together, these two axes of artificiality—Degree and intent of modification—may help explain the regulatory protection afforded to various aquatic ecosystems (Figure 1) in U.S. water law [23,29,30]; increasing modification with increasing intent renders waterbodies less likely to be protected [38]. Scientists, regulators, legislators, and other policymakers often do not explicitly acknowledge the value judgement implied by differential treatment of waterbodies according to their type of artificiality. Other traits such as technology, purpose, age, size, and permanence may also figure into value judgments and policy decisions people make about aquatic systems, and so might serve as a basis for further regulatory classification of artificial waterbodies. Some of these attributes, like small size and impermanence, may disproportionately characterize artificial aquatic systems, but also apply to most natural waterbodies [6,39]. Regulatory standards that omit smaller, less permanent waterbodies may do so as much because of their biological, geomorphological, and chemical features [23], or due to their dense distribution inconveniences property and land use considerations [40], as because of their human origins per se.

### **3. The Ecological Significance of Artificial Aquatic Systems**

Understanding the ecological and socio-ecological value of artificial aquatic systems requires that we understand their extent and distribution, their physical and chemical condition and how they relate to biotic communities, and the range of ecosystem services that they provide, but considerable uncertainty surrounds all of these characteristics [8]. Artificial aquatic systems are likely to be ecologically important, due to their extent, which may rival that of natural drainage systems and waterbodies. The ecological functions of artificial systems likely have social significance, often as ecosystem services and disservices, due to their frequent placement near large numbers of people. Moreover, the extent, distribution, and characteristics of artificial waterbodies are likely changing rapidly, in conjunction with those of natural waterbodies. Interdisciplinary understanding of the services and disservices of artificial aquatic systems, the factors that influence them, and their distribution in space and time could foster decisions that increase their ecological value.

### *3.1. The Extent and Dynamics of Artificial Aquatic Systems*

Our understanding of the extent of artificial aquatic systems is piecemeal. Available estimates are largely limited to the U.S. and other developed countries, and largely for intentionally designed aquatic features that are ubiquitous in agricultural, industrial, urban, and recreational land uses, but not their accidental counterparts, including in forests. Even with these incomplete inventories, it is clear that the deliberately constructed or altered fraction of the hydroscape is both large and growing, and must be included in any comprehensive assessment of aquatic resources.
