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Dam removal is potentially a powerful tool for river and riparian restoration. However, long-term studies on the fate of former reservoirs do not exist, limiting assessment of the utility of dam removal as a means of riparian restoration. We took advantage of the decades-long legacy of dam removals in Wisconsin to determine human uses of drained reservoirs and to evaluate vegetation establishment and species replacement at these sites. More than half of the 30 dam removal sites in southern Wisconsin over the past 47 years were used as commercial areas, parks and agricultural land, and active riparian restoration occurred on only two sites. For the 13 sites that were allowed to revegetate on their own, plants established in the first growing season and cover was very high at all sites in 2001. Species diversity and frequency (defined as percentage of sampled quadrats where a species is present) of trees were positively correlated with time since removal. No relationship existed between site age and frequencies of other growth forms, nor were there significant relationships between site age and the number or frequency of introduced species. However, mean frequency of introduced species was 75% per site and several sites were dominated by the introduced grass Phalaris arundinacea. Frequency of P. arundinacea was negatively correlated with number of native forbs, and lowest species diversity occurred on sites dominated by P. arundinacea. Ordination analyses revealed substantial site-to-site variation in vegetation that was weakly associated with gradients of site location, age, area, and soil phosphorus. Thus, temporal vegetation dynamics following dam removal were site-specific. Rapid revegetation demonstrates the potential of these sites for riparian restoration. However, if dam removal is used as a means of restoring native riparian communities, then approaches must be tailored to individual sites and will need to focus on techniques to minimize establishment of aggressive invading species.

Increased awareness of the negative effects of dams combined with an aging dam infrastructure has led to an increase in dam removals. However, ecological responses of downstream biota to such actions are poorly understood. We examined the influence of a pair of small dam removals on downstream periphyton and macroinvertebrates in Boulder Creek, WI (USA). The dams were 180 m apart and both were removed on 9 July 2003. We monitored algae and macroinvertebrates as well as habitat characteristics for approximately 2 months before and after the removals upstream and downstream from the two dams, and continued our observations over a similar period (mid-May to mid-July) the following summer. After the removals, an initial release of sediment significantly increased the proportion of fine sediments in the downstream reach and buried benthic substrate. This burial led to a 60% reduction in chlorophyll in the downstream reach the week following removal, while concentrations increased in the upstream reference reach. Similarly, macroinvertebrate densities 2 weeks post-removal were lower relative to pre-removal densities and were associated with declines of ephemeropterans, trichopterans and dipterans. Examination of Trichoptera genera demonstrated substantial changes in this assemblage associated with diminished densities of the formerly dominant genus Brachycentrus. Algal and invertebrate populations increased in the weeks after the dam removal, but did not reach densities similar to the upstream reference reach. In the following year, both periphyton and invertebrate densities were lower in the downstream reach, suggesting a long-term effect of the removal. Thus, effects of the dam removal were alternatively positive or negative over time, and indicate that the time scale of consideration has a strong effect on the interpretation of the consequences of this management activity.

[1] We generated suspended sediment solutions using river sediments and river water at concentrations similar to those observed during 1.5 year floods (Q 1.5 ) and a dam removal ($325 mg L À1 ) on the Deep River, North Carolina. Suspended sediment solutions were exposed to simulated solar radiation, equivalent to one clear, summer day at the study site (35°N). Concentrations of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), total dissolved nitrogen (TDN), dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), soluble reactive phosphorus (SRP) and total dissolved iron (Fe d ) were measured before and after exposure. Total dissolved carbon (TDC) budgets for each experiment were produced using DOC and DIC data. Sediment suspensions in the presence of simulated solar radiation were significant sources of dissolved C (119 ± 11 mmol C L À1 d À1 ; ± values indicate 1 standard error) and DON (1.7 ± 0.5 mmol N L

Dams create barriers to fish migration and dispersal in drainage basins, and the removal of dams is often viewed as a means of increasing habitat availability and restoring migratory routes of several fish species. However, these barriers can also isolate and protect native taxa from aggressive downstream invaders. We examined fish community composition two years prior to and two years after the removal of a pair of low-head dams from Boulder Creek, Wisconsin, U.S.A. in 2003 to determine if removal of these potential barriers affected the resident population of native brook trout (Salvelinus fontinalis). Despite the presence of other taxa in the downstream reaches, and in other similar streams adjacent to the Boulder Creek (including the brown trout, Salmo trutta), no new species had colonized the Boulder Creek in the two years following dam removal. The adults catch per unit effort (CPUE) was lower and the young-of-the-year catch per unit effort (YOY CPUE) was higher in 2005 than in 2001 in all reaches, but the magnitude of these changes was substantially larger in the two dam-affected sample reaches relative to an upstream reference reach, indicating a localized effect of the removal. Total length of the adults and the YOY and the adult body condition did not vary between years or among reaches. Thus, despite changes in numbers of adults and the YOYs in some sections of the stream, the lack of new fish species invading Boulder Creek and the limited extent of population change in brook trout indicate that dam removal had a minor effect on these native salmonids in the first two years of the post-removal.

River floodplains have the potential to remove nitrate from water through denitrification, the anaerobic microbial conversion of nitrate to nitrogen gas. An important factor in this process is the interaction of river water with floodplain soil; however, many rivers have been disconnected from their historic floodplains by levees. To test the effect of reflooding a degraded floodplain on nitrate removal, we studied changes in soil denitrification rates on the Baraboo River floodplain in Wisconsin, USA, as it underwent restoration. Prior to this study, the site had been leveed, drained, and farmed for more than 50 years. In late fall 2002, the field drainage system was removed, and a gate structure was installed to allow controlled flooding of this site with river water. Soil moisture was extremely variable among zones and months and reflected local weather. Soil organic matter was stable over the study period with differences occurring along the elevation gradient. High soil nitrate concentrations occurred in dry, relatively organic-poor soil samples and, conversely, all samples with high moisture soils characterized by low nitrate. We measured denitrification in static cores and potential denitrification in bulk samples amended with carbon and nitrogen, one year before and two years following the manipulation. Denitrification rates showed high temporal and spatial variability. Static core rates of individual sites ranged widely (from 0.00 to 16.7 microg N2O-N x [kg soil](-1) x h(-1), mean +/- SD = 1.10 +/- 3.02), and denitrification enzyme activity (DEA) rates were similar with a slightly higher mean (from 0.00 to 15.0 microg N2O-N x [kg soil](-1) x h(-1), 1.41 +/- 1.98). Denitrification was not well-correlated with soil nitrate, organic matter content, or moisture levels, the three parameters typically thought to control denitrification. Static core denitrification rates were not significantly different across years, and DEA rates decreased slightly the second year after restoration. These results demonstrate that restored agricultural soil has the potential for denitrification, but that floodplain restoration did not immediately improve this potential. Future floodplain restorations should be designed to test alternative methods of increasing denitrification.

[1] To determine how differences in geomorphologic setting influence spatial heterogeneity in transport and uptake of limiting nutrients, we investigated reach-scale interactions between porous bed material textures, bed morphology, transient storage, and nutrient retention in a field-scale flume (84 m  2.75 m). Conservative salt tracer and soluble reactive phosphorous additions were used to quantify effects of plane bed and alternate bar morphologies, and clean gravel versus sandy gravel bed texture on reach-scale nutrient retention and transient storage. We conducted experiments under light and dark conditions to clarify the role of benthic production on surface-subsurface hydrologic interactions and the relative influence of increasing biomass on nutrient uptake rates. Mean water residence time varied by a factor of 8 across treatments (4-32 min) and transient storage volume varied strongly with sediment texture. The exchange rate coefficient was greatly influenced by presence of alternate channel bars. Phosphorus uptake had the tendency to change with total volume of sediment-water interaction during dark conditions where periphyton abundance was low. However, under light conditions, periphyton growth clogged bed material pores and essentially eliminated exchange between the surface and subsurface. Uptake then was related to periphyton biomass accumulation rather than hydraulic or geomorphic parameters. The location and mechanism of stream nutrient retention may be more temporally and spatially dynamic than previously realized. Under clean bed conditions in streams (e.g., shaded riffles, streambeds following floods or in winter), nutrient uptake will be hyporheic dominated. Under high periphyton biomass conditions, nutrient uptake will be elevated in the surface sediments, minimal in the hyporheic, and thus benthic dominated.

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