What is this geographical feature? (as seen in maps)
The dark color of the water masses make me think they are peatlands in tundra.

Peatland is a terrestrial wetland ecosystem in which the production of
organic matter exceeds its decomposition and a net accumulation of
peat results.
The largest known concentrations of peatland are found in Canada and
Alaska, Northern Europe and Western Siberia, Southeast Asia, and parts
of the Amazon basin, where more than 10% of the land area is covered
with peatlands.

Source: peatlands.org