Fig. 1: Spatial distribution of stream sites by categorical groundwater signature.

Categorical groundwater (GW) signatures derived from annual paired air–stream water temperature signals a across the continental United States and b within a single watershed, the North Fork of the Clearwater River-Lake Creek watershed, Idaho—Montana, USA (Hydrologic Unit Code HUC10 – 1706030701). Lake Creek stream is highlighted. Across the United States, counts of each category are atmospheric signature (pink) n = 868; shallow groundwater GW signature (yellow) n = 292; deep groundwater GW signature (blue) n = 264. Legend descriptions are maintained between a and b.  Base map a was generated from R package ‘maps’ version 3.3.0 and the Nation Hydrography Dataset⁷⁰ b was created from 7.5-minute ground surface elevation data courtesy of the U.S. Geological Survey.