We examine the freshwater balance of Hudson and James bays, two shallow and fresh seas that annually receive 12% of the pan-
Arctic river runoff. The analyses use the results from a 3–D sea ice-ocean coupled model with realistic forcing for tides, rivers,
ocean boundaries, precipitation, and winds. The model simulations show that the annual freshwater balance is essentially between
the river input and a large outflow toward the Labrador shelf. River waters are seasonally exchanged from the nearshore region to
the interior of the basin, and the volumes exchanged are substantial (of the same order of magnitude as the annual river input). This
lateral exchange is mostly caused by Ekman transport, and its magnitude and variability are controlled by the curl of the stress at
the surface of the basin. The average transit time of the river waters is 3.0 years, meaning that the outflow is a complex mixture of
the runoff from the three preceding years.