Start with the exact parameter. A streamflow threshold in cubic feet per second cannot be applied to gage height in feet. Choose the station whose relationship to your place is useful and read its limitations.
Then decide whether you care about a high crossing, a low crossing, or a daily summary. Immediate alerts are useful for a small number of carefully chosen rules; daily digests reduce noise when timing is less urgent.
Use a re-arm buffer
If a rule triggers above 5 feet, a sensor oscillating between 4.99 and 5.01 should not send a message every few minutes. LakeRiverNow disarms the rule after the first event and re-arms only after the value clears your hysteresis buffer. Each event also has an idempotency key, so retries cannot create duplicate deliveries.
Thresholds are personal monitoring aids, not official flood stages or safety advice. Official alerts remain separate and higher priority.