A salmon’s otoliths—tiny bones in the head that help a fish maintain balance—contain a diary of its movements and a timeline of changes in diet. We use the chemistry recorded in daily bands (similar to tree growth rings) to track organisms that are too small to tag as they move among rivers, floodplains, estuaries, and the ocean over their lifetime. Strontium isotopes (87Sr/86Sr) in otoliths and carbon (δ13C), nitrogen (δ15N), and sulfur (δ34S) isotopes in salmon eye lenses provide natural markers to reconstruct time-resolved habitats critical to salmon growth, survival, and reproductive success. These markers allow us to develop isotopically distinct habitat maps that can be used to quantify year-to-year changes in the relative importance of different salmon rearing habitats.  An area of particular focus is quantifying the role of floodplains to the survival of endangered winter run and natural-origin fall-run Chinook salmon on the Sacramento River.