Soil and live plants act as a sponge, soaking up and cleansing the intercepted rainwater.
The water is held in the soil for later evaporation or is released slowly, like in the natural system, where rivers and streams are fed a fairly continuous supply of water even in dry spells. In the city stormwater system, a slow release of water is much less of a burden on the sewage treatment plants and this helps to reduce the amount of raw sewage overflows into our Willamette River. Also, the lower temperature of ecoroof runoff is Salmon-friendly.

Even ecoroofs have some runoff in heavy storms.
Another technique to reduce the stormwater burden is to use rainbarrels to hold the roofwater for later use in watering gardens or lawn, or for retention and eventual slow release after the storm has passed. This allows the stormwater to soak into the ground, where it becomes part of the natural hydrologic cycle rather than being flushed down the stormdrain. Our rainbarrels also help us measure the amount ot stormwater absorbed by our ecoroof. Tom Liptan of BES provided the dual-purpose design.

Very little of our stormwater will enter the storm sewer.
Most of it will be evaporated to the atmosphere or charged to the groundwater, eventually seeping into the Willamette River.

Our storm sewer leads rainwater to larger and larger pipes, combines with the city sewage system and reaches the sewage treatment plant.
Generally when it rains more than one tenth of an inch, the plant is flooded and releases raw sewage into the Willamette River. Some cities have separated their stormwater drains from the sewage system. This avoids the raw sewage problem, but does still does not provide for healthy Salmon-friendly watersheds and rivers.

On-site stormwater systems like ours support healthy watersheds, healthy rivers, and healthy Salmon populations.