The survey does not sample wherever volunteers happen to go. A scientific algorithm delineates stream reaches in advance, allocates them across the national river network, and releases them as claimable assignments. This page describes how those sites are chosen.
Every assignment is a highlighted segment of stream, typically a few hundred meters long, rather than a single coordinate. Within that segment a volunteer picks any safe bank-access location. The design keeps its geographic intent; the volunteer keeps the flexibility to stay safe.
Reaches are drawn from national hydrography and stratified by ecoregion, drainage position, elevation, flow regime, land cover, public-access status and existing monitoring coverage.
Allocating sites by stream miles alone would put the overwhelming majority of the survey in headwater channels, because that is where nearly all of the country's stream length is. Weighting each stream order by its Strahler order pulls effort back toward the larger streams and rivers that carry more of the network's biological signal.
Applied to a 100,000-site survey, that shifts the allocation substantially toward larger streams:
Order 1 and 2 streams still account for over half the survey — they remain the great majority of the network — but orders 4 through 9 grow from roughly 11% of sites to about 29%. Large-river mainstems receive a separate sampling framework on top of this allocation, since watershed units alone undersample major channels.
Reaches released from this design. They support nationally representative analyses when they meet every verification and quality requirement.
Sites proposed by individuals and groups outside the design. They add local biodiversity information and stay analytically distinct from the probability-based national dataset.