Restoring reservoirs following dam removal: best practices

How cool is it to come across studies looking at best practices for restoration following the removal of dams! We live in an era of incredible opportunity for ecosystem restoration and perhaps nowhere is that more true than the US Pacific Northwest where some 2000 dams are under review for potential removal.

Glines Canyon Dam slated to be removed in 2012. Image credit, American Rivers.However, it's not enough to remove the dam and hope for the best. A tremendous amount of planning and work is involved in making sure that dewatered reservoir sediment and riparian zones regain functionality and that we avoid problems such as soil erosion, exotic species invasion, and further degradation of water quality.

A new study in the Journal of Restoration Ecology tests various restoration practices for dealing with these issues. Researchers specifically looked at the effect of planting native species, applying mycorrhizal inoculum and using mulch in the restoration of plots containing dewatered reservoir sediment.

Study tests restoration methods on reservoir sediment...
Scientists from the University of California, Davis and the US National Park Service conducted the study on raised plots using sediment collected from Lake Mills, a reservoir behind the 64-m high Glines Canyon Dam in Washington. Glines Canyon Dam is slated to be removed in 2012. The project will be the largest dam removal completed to date and will entail restoring 50 million cubic meters of storage capacity currently inundated by Lake Mills.

The researchers compared 4 plots with different treatments: (1) unplanted sediment with no inoculum; (2) native seeds and transplants with no inoculum; (3) native seeds and transplants with a commercial mycorrhizal inoculum; and (4) native seeds and transplants with mulch sacks of alder chips inoculated with saprophytic spores in addition to mycorrhizal inoculum. The researchers were particularly interested in finding out which methods would enable establishment of mycorrhizal bacteria - a key for plant growth because the bacteria form symbiotic relationships with plants helping them to take up critical inorganic soil resources like nitrogen and phosphorus.

Findings you might not expect...
Surprisingly, the researchers found that most plants in both uninoculated and inoculated plots became mycorrhizal after 20 months, demonstrating that no treatments were needed to establish mycorrhizal colonization. They also found that the unplanted plot had the greatest number of native volunteer species establishing - an expected result (volunteer species are those which show up on their own). But among the planted plots, the treatment with mulch had the greatest number of native volunteers, perhaps because the mulch retained moisture creating better conditions for establishment. The planted plots with additional mycorrhizal and mulch treatments had substantially lower exotic plant abundance than the unplanted and planted plots without mycorrhiza or mulch. All plots had improved soil aggregate stability, but the mulched plot had much greater reduction in runoff and erosion.

Based on these results the researchers drew the following conclusions for restoration practices:

Implications for practice:
• Natural mycorrhizal inoculation through wind or animal spore dispersal from neighboring ecosystem source areas can sufficiently colonize native plant species over time without requiring commercial inoculum at the plot scale.

•Reservoir-scale restoration may greatly benefit from active establishment of key plant and fungus source areas to achieve rapid native ecosystem restoration.

•Increasing soil moisture retention through placement of mulch sacks may significantly improve establishment and growth of native vegetation.

•Rhizosphere activity, including mycorrhizal colonization, will reestablish following reservoir dewatering, significantly improving soil aggregate stability over time.

•Revegetation coupled with mulch sacks will reduce runoff and erosion 10-fold compared to plots with no planting or mulch sacks.

**Note** Front page photo of Glenn Canyon Dam. Image credit, Paul R. Kucher.

Source:	Restoration Ecology
Title:	Effects of Native Plant Species, Mycorrhizal Inoculum, and Mulch on Restoration of Reservoir Sediment Following Dam Removal, Elwha River, Olympic Peninsula, Washington
Authors:	a) Kerri Cook, b) Wesley Wallender, Caroline Bledsoe, Gregory Pasternack, and Shrini Upadhyaya
	a) North Cascades National Park Service Complex; b) University of California, Davis