Outdoors group says Geological Survey underestimates number of ephemeral streams; see interactive map

Trout Unlimited has submitted data to show that the U.S. Geological Survey underestimates the number of streams that only flow after rain.

"The fishing and conservation group says that for every mile of stream mapped in the National Hydrography Dataset, another 1.5 miles of ephemeral streams exist," Ariel Wittenberg reports for Energy & Environment News. "The analysis comes as the Trump administration is soliciting comments on its Waters of the U.S., or WOTUS, rule that would eliminate Clean Water Act protections for ephemeral streams, which flow only following rainfall."

The National Hydrography Dataset shows that 18 percent of U.S. streams are ephemeral, and stand to lose federal protection. Trout Unlimited and other outdoors groups oppose the new rule, saying that ephemeral streams are an important habitat for many fish species and provide critical supplies of clean water to other habitats downstream, Wittenberg reports.

Mapping specialists working for Trout Unlimited looked at other studies of when and where ephemeral streams form. "That science showed that ephemeral streams initiate in steep areas where more than 2 acres of a watershed drain to a particular point. In flat areas, ephemeral streams initiate when roughly 24 acres of watershed drain to a particular area," Wittenberg reports. "So Trout Unlimited estimated that 'unmapped' ephemeral streams existed in areas where more than 11 acres of watershed would be draining." 

The scientist leading Trout Unlimited's efforts, Kurt Fesenmyer, said the approach was conservative, and would generally lead to overestimating the number of ephemeral streams in flat places and underestimating them in wet areas. Trout Unlimited created an interactive map and state-specific estimates. Click here to see them.

The Environmental Protection Agency and the Army Corps of Engineers wrote in their economic analysis of WOTUS that the National Hydrography Dataset should not be used because it mostly doesn't generally differentiate between permanent and ephemeral streams, Wittenberg reports.