Organic Matter in Fanno Creek, Oregon: Where It Comes From and Where It Goes

Organic matter is what remains of living things — fallen leaves, yard waste, animal waste, fallen timber and other plant and animal remains — on land and in water. It feeds the microbes at the base of the food chain, carries some trace metals and affects how light passes through water. In a stream it turns up in leaf litter, algae, soil, and particles suspended or dissolved in the water.

It can also harm a river: microbes breaking it down use up dissolved oxygen, stressing fish and other animals. Fanno Creek, in the Portland, Oregon, metropolitan area, is listed as impaired by the Oregon Department of Environmental Quality and must be managed to raise its dissolved oxygen to state standards, which vary with the season and with whether fish are rearing or spawning. The U.S. Geological Survey, working with Clean Water Services, studied where the creek's organic matter comes from, how it moves and where it ends up, to help managers cut the excess that lowers oxygen in Fanno Creek and, in summer, downstream in the Tualatin River.

From the trees

For Fanno Creek, most organic matter comes from land — leaf litter and soil — not from algae. Using field measurements and lidar, USGS scientists estimated the creek's foliage biomass, the dry weight of all its leaves: about 990 metric tons a year over the floodplain, about 140 tons of it directly over the channel. How much falls depends on the trees' size and age and how dense the canopy is; forested reaches, with more mature broadleaf trees, produce more than wetland or urban reaches.

From the banks

Measuring carbon in bank samples and rates of erosion and deposition, the team estimated that streambanks supplied 50 to 120 tons of organic matter a year — less than the leaves above. Topsoil deposited on the floodplain was 2 to 5 times richer in carbon than subsoil from the banks.

Net deposition was greatest along the middle reaches, where wetlands have been restored. Geomorphic maps of the floodplain show where sediment is eroding and depositing — useful for anyone planning wetland or riverside restoration. About 70 percent of the banks looked highly erodible, and the creek exported 550 to 3,300 tons of sediment during the year-long study.

In the water

With optical fluorescence sensors in the stream and laboratory fluorescence and isotope analyses, the scientists found that about 320 tons of organic carbon pass through Fanno Creek each year, about 70 percent dissolved. Dissolved organic carbon generally stays at or above 3–4 milligrams per litre for most of the year. The results confirmed that the stream's carbon comes mainly from land sources such as leaf litter and surface runoff.

The first storm of autumn

In mid-October 2012 the first autumn storm stirred up fine particles that had built up on the streambed through spring and summer and washed in more from storm drains and soils. Real-time monitors caught it. Because of that stored material, a large share of the year's organic matter moved in this one storm, even though bigger storms came later. The material was also different: more decomposed by microbes, confirming that easily moved sediment and rotting organic matter had collected on the bed during the dry months. Downstream, the storm's plume caused a measurable, significant drop in dissolved oxygen in the Tualatin River — something that tends to happen with the first autumn storms most years.

Why it matters

Fanno Creek runs through a heavily urbanised area with many stakeholders — cities, counties, utilities, conservation groups and residents. Its health matters to the people who live and play along it as well as to aquatic life, and the study's maps, data and findings are meant to help managers decide on management and restoration.

Sources

• Sobieszczyk, S., Keith, M.K., Goldman, J.H., and Rounds, S.A., 2015, Organic matters — Investigating the sources, transport, and fate of organic matter in Fanno Creek, Oregon: U.S. Geological Survey Fact Sheet 2015–3003, prepared in cooperation with Clean Water Services. https://pubs.usgs.gov/publication/fs20153003
• Full results: three articles in the Journal of Hydrology, 2014 (Sobieszczyk and others; Keith and others; Goldman and others).
• Figures and photos: U.S. Geological Survey, from the fact sheet's PDF; the erosion maps use Oregon Department of Geology and Mineral Industries elevation data as their base.
• Rewritten in hubnx's own words.