Rivers don't flush suspended microplastic straight downstream — the sticky biofilm mat coating rocks and sediment in slow pools grabs and holds a disproportionate share of it, while the same particles pass largely untouched through fast, scoured channel sections. This simulation grows biofilm over a 3D streambed at a rate set by local flow speed, drifts microplastic particles downstream through the flow field, captures them probabilistically wherever biofilm has thickened, and lets you trigger a storm event that scours the mat and its trapped load loose as a downstream pulse — with live biofilm coverage, per-zone trapped totals and downstream flux readouts.