Historical clue formulations

• Calm, as a lake — 2026-07-06
• Tranquil — 2021-12-09
• Even-tempered — 2021-06-27
• Tranquil — 2021-05-24
• Serene — 2020-11-04
• Not having so much as a ripple — 2019-10-17
• Even-tempered — 2009-09-29
• Opposite of turbulent — 2008-09-20
• Unruffled — 1998-04-16
• Lake name of two Olympics — 1997-02-09