Reading the map
thickness is how much water is moving
colour is how far above normal: 1× to 4×+
dotted means no gauge upstream, so flow is unknown. Zoom in above Springdale to see
the side canyons that feed the North and East Forks.
How the estimates work, and what they miss
River lines are the USGS National Hydrography Dataset (high resolution) channel, except Quail Creek, traced from satellite imagery.
Line thickness shows how much water a reach is carrying. Line colour shows how far above its own
quiet baseline it is, so a surge reads as a warm band moving down the system while the river keeps
its true size. Baseline is each gauge's 25th-percentile flow; a rise above it is routed downstream at
kinematic-wave speed (c = 1.67v, v ≈ 0.75·Q0.29 ft/s),
losing about 30% per 20 miles, and blended toward whatever the next gauge downstream reads.
A dotted hairline means no gauge exists anywhere upstream of that reach, so its flow is unknown —
it is never inferred from the main stem. Ungauged side canyons entering a gauged reach are invisible
to the model, and that is exactly where a monsoon flash flood comes from.
Plan with it; do not rely on it instead of an NWS warning.
Rain radar is the NWS NEXRAD composite, refreshed every five minutes. The nearest radar sits
near Cedar City, and terrain blocks its lowest beam over parts of Zion and the East Fork, so
light rain in deep canyons can go unseen. Colour runs from green for light rain through yellow
and red for heavy; the loop shows the last hour so you can see which way cells are moving.