Measurements of the diffusion flow of CO2 soil gas in the area of the Nizhne-Shchapinsky thermal springs (Central Kamchatka) have been carried out for the first time. The measured flux was about 7 ± 1.3 t/day from an area of 22 000 m2. A retrospective analysis of historical literature data and satellite images has shown that highly dynamic riverbed processes are taking place in the area of the Nizhne-Shchapinsky springs. The river banks are subject to significant morphological changes. Based on this, it can be concluded that the springs on this island are not new previously unknown formations, but modified ones that existed on the right-bank thermal site and were described in 1908. The chemical composition of the Nizhne-Shchapinsky thermal springs showed that there are two groundwater horizons: a deeper one of Na–Cl composition and a near-surface one of Mg–Ca–HCO3 composition, which are the sources of water supply for the thermal waters. The most likely way to form these waters is to mix about 10% of the deep sodium-chloride component with water from a relatively shallow reservoir, which is based on carbonate rocks, including magnesite, under conditions of intense saturation of groundwater with carbon dioxide.