Climate change in the northern circumpolar regions is rapidly thawing organic-rich permafrost soils, leading to the substantial release of dissolved CO2 and CH4 into river systems. This mobilization impacts local ecosystems and regional climate feedback loops, playing a crucial role in the Arctic carbon cycle. Here, we analyze the stable carbon (delta 13C) and radiocarbon (F14C) isotopic compositions of dissolved CO2 and CH4 in the Sagavanirktok and Kuparuk River watersheds on the North Slope, Alaska. By examining spatial and seasonal variations in these isotopic signatures, we identify patterns of carbon release and transport across the river continuum. We find consistent CO2 isotopic values along the geomorphological gradient, reflecting a mixture of geogenic and biogenic sources integrated throughout the watershed. Bayesian mixing models further demonstrate a systematic depletion in 13C and 14C signatures of dissolved CO2 sources from spring to fall, indicating increasing contributions of aged carbon as the active layer deepens. This seasonal deepening allows percolating groundwater to access deeper, older soil horizons, transporting CO2 produced by aerobic and anaerobic soil respiration to streams and rivers. In contrast, we observe no clear relationships between the 13C and 14C compositions of dissolved CH4 and landscape properties. Given the reduced solubility of CH4, which facilitates outgassing and limits its transport in aquatic systems, the isotopic signatures are likely indicative of localized contributions from streambeds, adjacent water saturated soils, and lake outflows. Our study illustrates that dissolved greenhouse gases are sensitive indicators of old carbon release from thawing permafrost and serve as early warning signals for permafrost carbon feedbacks. It establishes a crucial baseline for understanding the role of CO2 and CH4 in regional carbon cycling and Arctic environmental change.

Splays-fan-shaped depositional landforms produced by overbank deposition by unconfined flows-can damage structures, degrade arable land and incur substantial mitigation costs. Splay-related hazards along many rivers are likely to worsen with the increasing magnitude and frequency of major floods. The highly incomplete understanding of splays on braided streams is a conspicuous knowledge gap in a changing world with more frequent and intense floods. The largest recorded flood on the braided, sand-dominated lower Platte River (eastern Nebraska, USA) in March 2019 resulted from the rapid melting of a deep, moist snowpack during an extreme rain-on-snow, bomb-cyclone event. This flood produced 32 large (as much as 234 ha) splays that buried structures and cropland under sand. A total of 1,438 ha of row crop was buried, equating to 1.2 million dollars in lost revenue. These splays diverged from the channel by 14 degrees to 104 degrees along a 122 km reach. The topography of preexisting abandoned channels strongly controlled the shape and orientation of most splays, although forested areas tended to trap or divert sediment. The flood eroded 2.2 to 202 m(2) m(-1) of the streambank at 11 of the splays. The five largest splays (>100 ha) deposited as much as 2.4 m of sand. Ground-penetrating radar profiles of the largest splay indicate that it consisted almost entirely of overbank deposits exhibiting simple downstream accretion that buried the pre-flood soil under <= 1 m or less of sand. Locally, however, this soil was eroded during the flood. Climate models predict increasing winter precipitation in the Platte River basin; therefore, the frequency of major floods should increase, making splays recurrent hazards. Our geomorphic assessment of the splays on the lower Platte River illustrates the need for future hazard and mitigation planning.