usgs-waterUSGS National Water Information System (NWIS) - real-time and historical water data. Streamflow, gage height, groundwater levels, and monitoring site information. CRITICAL USAGE RULES: 1. No authentication required 2. searchSites takes exactly ONE location filter: bbox ("west,south,east,north") OR countyCd (5-digit FIPS) OR stateCd (2-letter). Empty result = no sites matched. 3. Use site IDs from searchSites() for data queries (getGroundwaterLevels also accepts bbox directly) 4. Parameter codes: 00060=discharge, 00065=gage height, 72019=depth to water 5. Period format: P1D=1 day, P7D=7 days, P30D=30 days. getDailyValues defaults to P30D when no dates given.