Grass or rock in the channel
Written 1 Sep 2026. The grass camp: plants filter, cool, and hold soil, and a mowed swale looks like a lawn feature instead of a construction site. The rock camp: grass dies under standing water and scours under fast flow, while stone takes both and asks for nothing. Both camps are describing the same two failure modes from opposite ends — water that sits too long, and water that moves too fast.
The ledger
| Condition | Grass | Rock |
|---|---|---|
| Standing water > 48 h | Dies, then the channel scours bare | Indifferent |
| Flow over 1.5 ft/s | Scours | Holds, if sized — riprap |
| Filtering runoff | Filters well | Does almost nothing |
| Mowing | Mowable | Weeds through the voids, forever |
| Silt | Traps it in the roots | Traps it in the voids — the voids are the point |
| Mosquito posture | Wet grass = habitat | Dry surface, same rule applies — 72 hours |
The desk's rule
Grade and ponding decide, not taste. Below 2% grade with water that clears in 48 hours, grass wins — it filters and mows. Over 3–4% grade, or where water stands past 48 hours, rock wins, and the grade likely wants check dams anyway. The hybrid — rock in the wet line, grass on the slopes — is the most common working answer and the hardest to mow cleanly. State the maintenance before choosing it; the dam page says the same about silt, and both bills come due.
The disagreement sentence
Rock-lined swales are sold as maintenance-free. They are not: the weeds and the silt are the maintenance, and they arrive annually.
Where to go next
- Swale — the channel being lined.
- Plant a swale — the grass option, done right.
- Riprap — the rock option, sized.
Sources
- USDA NRCS — Grassed waterway (code 412), conservation practice standard — supports vegetated waterway requirements and where vegetation fails.
- Minnesota Stormwater Manual — Design criteria for wet swale (wetland channel) — supports the manual's guidance on lining a swale.
- Minnesota Stormwater Manual — Infiltration — supports what happens to infiltration when a lining is added.