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Peatlands are essential for biodiversity conservation in wetland ecosystems and for mitigating global warming through carbon storage. Given the importance of peatlands, conservation and restoration efforts have been undertaken worldwide. This study examined the Kushiro Mire in Hokkaido, Japan, where river channel straightening in the 1980s led to wetland degradation, including the conversion of the mire to alder stands and the loss of the wetland landscape. In the 2010s, a nature restoration project was implemented to restore the river meander of the Kushiro River. In this study, tree-ring core samples were collected from alder trees growing around the restored river meander area. They were used to estimate tree age from the number of tree rings and annual growth from the ring width. The effects of river restoration on alder growth were then assessed based on changes in growth. Eight alder tree-ring surveys were conducted at sites where tree-ring conditions were expected to differ. At each site, five to six trees exhibiting typical growth conditions were surveyed. Tree-ring analysis revealed that the average tree ages at the eight sites were roughly divided into two groups: 25 and 35 years, with a difference of approximately 10 years. Both groups had entered and expanded within the mire following the 1984 river channel straightening, but before the river meander restoration was completed in 2011. At survey points near the restored meandering river channel, there was a statistically significant decrease in tree-ring growth following restoration relative to pre-restoration conditions. Conversely, at survey points near the former straight river channel, there was a significant increase in tree-ring growth following restoration. Flood inundation simulations with pre- and post-restoration of the river meander implied an increase in sediment thickness after restoration at survey points near the former straight river channel. Meanwhile, at survey points downstream of the restored meandering river channel, flood-induced sediment deposition was implied to decrease. The simulation results indicate that flood-induced sediment inflow into the mire could alter nutrient distribution, contributing to alder growth.