Trace controls
Longer all-tributary traces can return many segments. The map never runs an unconstrained trace.
What this means
The lines show hydrologic network connectivity in NHDPlusV2. “Upstream” means reaches connected toward contributing headwaters; “downstream” follows the modeled mainstem away from the selected reach. This is not a contaminant travel-time model, flood forecast, discharge estimate, or proof that water is currently flowing through every displayed segment.