Storm & Flood Damage Recovery in Bellevue, WA

That is a soil boundary rather than a spring. Permeable material sits over dense till across much of this area; rain soaks down through the upper layer, cannot pass the till, and then travels sideways until it daylights partway down a slope. A house built at that contact takes water through its uphill wall under pressure. Interception uphill with a curtain drain is the standard answer, and it needs designing for the specific slope.

Sustained winter saturation does that, and it is worth documenting rather than watching casually. Photographs from fixed points across a season show whether the bank is moving and how fast, which is what a geotechnical assessment needs. Increased flow from upstream development is a common driver. It is a slow process, which means there is usually time to act, and also that it is easy to leave until it is expensive.

Not really, and that has to shape expectations. Rain arrives continuously rather than in events, so materials never get a recovery period and a small defect runs all winter instead of drying between wettings. Outdoor air does no drying either. Every job here should assume a closed system with dehumidification, and a crawl space is far more informative inspected in February than in August.