Citation: Fang, B.; Kam, J.; Elliott, E.; Tootle, G.; Therrell, M.; Lakshmi, V. The Recent Decline of Apalachicola– Chattahoochee–Flint (ACF) River Basin Streamflow. Hydrology 2022, 9, 140. https://doi.org/10.3390/ hydrology9080140 Academic Editor: Monzur A. Imteaz Received: 21 June 2022 Accepted: 24 July 2022 Published: 5 August 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). hydrology Article The Recent Decline of Apalachicola–Chattahoochee–Flint (ACF) River Basin Streamflow Bin Fang 1, *, Jonghun Kam 2 , Emily Elliott 3 , Glenn Tootle 4 , Matthew Therrell 3 and Venkat Lakshmi 1 1 Engineering Systems and Environment, University of Virginia, Charlottesville, VA 22904, USA 2 Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea 3 Department of Geography, The University of Alabama (UA), Tuscaloosa, AL 35487, USA 4 Department of Civil, Construction, and Environmental Engineering, The University of Alabama (UA), Tuscaloosa, AL 35487, USA * Correspondence: bf3fh@virginia.edu Abstract: The Apalachicola–Chattahoochee–Flint (ACF) basin is arguably the most litigated interstate river system in the eastern United States. Given the complicated demands for water use within this basin, it has been difficult to ascertain if the recent multi-decadal decline in streamflow is a product of human disturbance, changing climate, natural variability, or some combination of the above factors. To overcome these challenges, we examined unimpaired streamflow and precipitation within and adjacent to the ACF basin, upstream of the Apalachicola River at Chattahoochee, and the Florida streamflow station (ARCF), which has historically been identified to be representative of hydrologic variability in the ACF basin. Several of the upstream, unimpaired, streamflow stations selected were identified in rural watersheds where land-cover changes and human disturbance were minimal during the study period. When applying a series of statistical evaluations, ARCF streamflow variability generally reflects the natural variability of the ACF basin. Additionally, unimpaired streamflow variability from the neighboring Choctawhatchee River compared favorably with ARCF variability. The recent multi-decadal decline was consistent in all records, with the 2000s being the most severe in the historic record. Keywords: streamflow decline; hydrologic variability; precipitation temporal variation 1. Introduction The interstate waters that flow through the Apalachicola–Chattahoochee–Flint (ACF) river basin provide a crucial resource to the southeastern United States. The ACF basin contributes to water demands of metropolitan Atlanta, instream flow for agriculture in southwest Georgia, energy production in Alabama, and the essential freshwater needed for Florida’s shellfish industry. A combination of multiple droughts over the past three decades in the southeastern U.S., along with immense population growth in the region, has resulted in a strain of this much needed and once abundant resource—water—resulting in litigation around sharing ACF waters. While conflict around water resources has been contested in the western U.S. (aka the Colorado River) for over a century, the southeastern U.S. has been largely isolated from this problem due to abundant instream flow availability. However, the Tri-State Water Wars between Alabama, Florida, and Georgia over the waters of the ACF have been waged in U.S. courts since 1990, reaching the U.S. Supreme Court on multiple occasions. With the increased demands for water within the basin, as well as the multi-decadal trends in declining streamflow throughout the region, it has been difficult to parse the complicated interactions between human disturbance, changing climate, and natural hy- droclimatic variability within the region. Large-scale climate forcing mechanisms, such as the high-frequency El Nino Southern Oscillation (ENSO) and the low frequency Atlantic Hydrology 2022, 9, 140. https://doi.org/10.3390/hydrology9080140 https://www.mdpi.com/journal/hydrology