Understanding Runoff: Why It Matters During fall, Houston typically experiences increased rainfall. When lawns are treated with synthetic fertilizers, herbicides, and pesticides , those chemicals don’t stay put. Rain washes them off lawns, driveways, and garden beds — sending pollutants straight into storm drains , and ultimately, into Houston’s bayous and waterways . This runoff can: Deplete oxygen levels in the water (hurting fish and aquatic life) Contaminate drinking wate