Water spreads out from the river channel to every connected spot lower than the estimated water level for that stretch of river. Levels come from the cross-sections at each point and are blended in between; above the Johnson Fork mouth the Llano uses the Junction gauge itself. Ground comes from USGS lidar resampled to about 12 m, so small ditches and low spots can be missed or merged. Buildings and fences are not in the ground model. Treat the edges as approximate.
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Each view looks downstream across the river at that point, drawn from 1-meter USGS lidar ground elevations (vertical scale exaggerated). Water levels come from the channel shape and slope, with roughness tuned so the Junction gauge section matches today's real rise within about 0.7 ft from 9,500 to 34,600 cfs. Lidar can't see below the water surface, so levels near normal flow are approximate; flood levels are more reliable. These are estimates, not survey measurements.
Feet are readings at each gauge (normal feet come from the last three years of LCRA records), not water depth at the crossings. Downstream estimates add the Junction and Johnson Fork readings, each shifted by its travel time. Past the last reading, Junction follows the NWS forecast and Johnson Fork is assumed to recede by half every 6 hours. Creek inflow below Johnson Fork is not counted, so during local rain the real river will run higher than these numbers.
| Flow at Junction | to rm 7.3 | to Grobe | to Yates | to Mason |
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Flood waves on the Llano move faster when the river is high. Matching 30 flood peaks between the Junction and Mason gauges (2002–2026, 52.5 river miles), with the James River and Johnson Fork water subtracted at Mason, gives a travel time of 52.3 − 8.94 × log10(flow) hours, within about ±3 hours. Shorter reaches are scaled by measured river miles below the Junction gauge: Johnson Fork mouth 5.60, Colvin Casa 7.56, Holly & Tommy's 9.07, Grobe 11.32, Yates 16.52; Johnson Fork @ Pipeline ~7.5 below the Johnson Fork gauge, which sits ~9 river miles above the fork's mouth. Field observations are used to tune these numbers.