A river reach's water quality is a running contest between pollution load coming in and how much restoration and natural recovery can process out.
quality(t+1) = quality(t) + recovery*investment - pollution_load
- River grid length — river reaches tracked independently for water quality.
- Restoration investment — share of reaches receiving active riparian restoration or pollution control.
- Pollution load — agricultural runoff and wastewater pressure entering the river.
- Natural recovery rate — how quickly a restored reach's ecosystem processes pollutants on its own.
River restoration projects that combine riparian planting with upstream pollution controls have brought long-dead urban rivers like the Thames back to healthy fish populations within decades.