Rain today does not imply that a river will change in the same way today. Where the rain fell, what happened in the soil, which path the water followed, and how long that path took all contribute to the river's response.
A river name can stay the same while the measurement point changes. Long-term comparison requires knowing which point, source, method, and units belong to each part of the history.
People living near a river usually want to know how high the water may rise. A model may instead provide discharge. Flow and water level are related, but one cannot be read directly from the other without local river geometry and conditions.