3.1.1. Deliberately Modified Waterbodies

Deliberately constructed and transformed channels constitute a significant portion of the U.S. hydroscape (Figure 2). The U.S. National Hydrography Database includes 5525 km of ditches

and canals, or approximately the length of the Missouri River through the Mississippi to the Gulf of Mexico. This aggregation is probably a substantial underestimate, as many smaller ditches do not appear in the database. In 2010, ditches in U.S. agricultural lands occupied about 115,760 km<sup>2</sup> [41], a surface area similar to that of Lakes Superior and Huron combined, or 2.67 times the surface area of all U.S. streams combined [39]. Channelization, another deliberate transformation, has altered the geomorphology of upwards of 26,550 km of rivers and streams in the U.S. [31,42], more than seven times the length of the Mississippi River. Humans have channelized more than 500,000 km of rivers worldwide, and built more than 63,000 km of canals [43–45]. The U.S. also has 6.5 million km of roads [46], many of which have ditches or gutters along both sides that contain water at least occasionally and often for much longer periods. Thus, it is likely that road drainage in the U.S., which effectively serves as urban headwaters, is of comparable length to the 5.3 million km of the country's rivers and streams. The U.S. EPA estimates that 77% of the approximately 1.8 million km of wadable streams in the U.S. are in poor (42%) or fair (25%) condition [47]. Some of these poor and fair streams are likely geomorphically modified enough to fall within our gradients of artificiality. Within this inventory, the remaining degraded "natural" streams, and even perhaps "natural" streams in good condition as well, likely occupy less length than artificial channels. Known lengths of "natural" streams, especially ephemeral and intermittent ones, likewise underestimate their extent [8].

**Figure 2.** Extent of artificial as compared to natural waterbodies in the U.S. Data drawn from a variety of sources, shown in parentheses. (**a**) areal features; (**b**) linear features. Data are taken from the National Hydrography Dataset (NHD+) [48], U.S. Army Corps of Engineers National Inventory of Dams (USACE) [49], U.S. Department of Agriculture Natural Resources Conservation Service (NRCS) [41], Downing et al. [6], U.S. Fish and Wildlife Service (USFWS) [50], Research and Innovative Technology Administration (RITA) [46], and Leopold [42].

The extent of constructed lakes and ponds are similarly significant in comparison to natural waterbodies (Figure 2). The U.S. has about 22,000 km<sup>2</sup> of deliberately constructed or transformed farm ponds an area similar to that of Lake Michigan; the world as a whole has about 76,830 km<sup>2</sup> of farm ponds [6]. In total, for the U.S., the U.S. Army Corps of Engineers inventories an impounded (deliberately transformed) area in excess of 180,000 km<sup>2</sup> [49], for purposes including irrigation, hydroelectricity, flood control, navigation, water supply, and recreation [49]. The world had approximately 258,570 km2, or slightly more than the area of the Great Lakes, of impounded water in the mid-2000s [6], before the completion of the Three Gorges Dam in China and other projects. In 2009, the U.S. Fish and Wildlife Service estimated that only 31% of the country's 27,151 km<sup>2</sup> of freshwater ponds were natural. Of the artificial pond area, about the size of Lake Ontario, farm ponds took up about 1.5 times the space as natural ponds; urban ponds occupied about half the area of natural ponds despite the relatively small amount of urban space, and industrial and aquaculture ponds made up significant fractions as well [50]. This estimated aquaculture pond area, of just over a thousand square kilometers, is much smaller than that of many countries'; globally, 1.7 million km<sup>2</sup> of the world's 2.7 million km<sup>2</sup> of irrigated land goes to rice production, and is flooded seasonally, at least [4].
