warming of water in spring
A seasonal phrase (kigo) used in haiku and poetic language to evoke early spring, when the water begins to lose its winter chill.
In the season when the water warms, it's pleasant to walk along the riverside.
In haiku, 'mizu nurumu' is often used as a spring seasonal word.
Compound of 水 (mizu, 'water') and 温む (nurumu, 'to become lukewarm'). The phrase is a classical seasonal term for early spring.