Jim Pizzuto

Comparing gravel-bed rivers in paired urban and rural catchments of southeastern Pennsylvania

Surveys in eight paired urban and rural watersheds illustrate how urbanization changes fluvial morphology and processes. Our data also provide quantitative criteria for evaluating streamrestoration projects in urban areas. Bankfull depth, reach-averaged bed slope, and median grain size are similar in urban and rural watersheds. The median width of urban channels is 26% larger than the median width of rural channels. The median sinuosity is 8% lower in urban channels and pools are 31% shallower. The median composite Manning’s n based on median grain diameter, pool depth, and channel sinuosity is 10% lower in urban streams, while the median bankfull discharge per unit drainage basin area is 131% higher in urban channels. Histograms of bed sediment-size distributions in urban channels lack a secondary mode in the size range 2‐64 mm characteristic of rural channels, indicating that these sizes tend to be selectively removed from urban channels. However, bankfull Shields stresses in urban and rural channels exceed typical threshold values at most sites, indicating significant bedload transport at bankfull stage. Apparently, increased peak discharges caused by decades of urbanization have not removed all the transportable sediment from these urban stream channels. We speculate that the supply of sediment to urban channels from hillslope processes and channel erosion remains significant, even though much of the upland surfaces of these urban catchments are covered with nonerodible impervious surfaces.

Influence of bank vegetation on channel morphology in rural and urban watersheds

Stream-bank vegetation significantly influences the morphology of streams in the Piedmont region of the United States. We surveyed the morphology of 26 paired stream reaches in southeastern Pennsylvania, northern Maryland, and Delaware. One member of each pair has a forested riparian zone, whereas the other has a riparian zone composed pri marily of grass. The paired reaches are nearly contiguous, so all significant channel-forming variables except riparian vegetation are held constant. The extent of urban development of the watersheds upstream of the paired reaches also varies considerably, allowing us to determine the combined influence of riparian vegetation and urbanization on channel morphology. Statistical analyses indicate that (1) channels with forested riparian zones are wider than channels with nonforested riparian zones, (2) channels in urbanized watersheds are wider than channels in nonurbanized watersheds, and (3) the effect of riparian vegetation is independent of the level of urbanization.

Increased mid-twentieth century riverbank erosion rates related to the demise of mill dams, South River, Virginia

A recent hypothesis suggests that fluvial processes in areas of the eastern United States are strongly influenced by the demise of colonial mill dams, rather than reflecting a quasi-equilibrium adjustment to the current hydrologic and sediment regime. We evaluated the control of colonial mill dams on twentieth and twenty-first century bank erosion rates on the South River, Virginia, through studies of historical aerial photographs, historical documents, hydrologic and climatic records, and hydraulic modeling. Historical sources and aerial photographs document eight colonial mill dams along the study reach in the early twentieth century; all but one of these dams disappeared in the 1950s, and the last was breached by 1976. From initially low values between 1937 and 1957, mean bank erosion rates increased by more than a factor of 2 after 1957, remaining high through 2005. Accelerated bank erosion rates cannot be explained by changes in storm intensity, the frequency of freeze-thaw cycles, or by changes in the density of riparian trees. Hydraulic modeling suggests that mill dams reduced velocities of the 5 yr flood through ~80% of our study reach. By considering the timing of mill dam loss, the spatial extent of backwater influence, and the locations of our study sites, we find that the loss of mill dams explains the observed trends in bank erosion rates at 9 (and possibly 10) of our 14 monitoring sites. These results support the hypothesis that the demise of mill dams has been an important influence on fluvial processes in the region.

Predicting the accumulation of mercury‐contaminated sediment on riverbanks—An analytical approach

[1]xa0Mercury was introduced into the South River, Virginia, as a result of industrial use from 1929 to 1950. To guide remediation, an analytical model is developed to predict the mercury inventory resulting from deposition of mercury-contaminated sediment on subhorizontal surfaces adjacent to the river channel from 1930 to 2007. Sediment cores and geomorphic data were obtained from 27 sites. Mercury inventories range from 0.00019 to 0.573 kg m−2. High mercury inventories are associated with frequent inundation by floodwaters, forested riparian vegetation, and (at only four sites) unusually high sediment accumulation. Over the 10 km study reach, mercury inventories do not vary with downstream distance. The frequency of inundation at each coring site is determined from hydrologic data and a streamtube stage-discharge model. Water levels are exponentially distributed. A simple parameterization represents the enhanced ability of forested vegetation to trap mercury-contaminated sediments compared to nonforest vegetation. The calibrated model explains 62% of the observed variation in mercury inventories; 15 of the 27 predicted values are within a factor of 1.8 of the observed values. Calibration indicates a mercury deposition rate during inundation of 0.040 kg m−2 yr−1 (95% C.I. 0.032–0.048), that forested areas accumulate mercury-contaminated sediment 3.05 (95% C.I. 2.43–3.67) times faster than nonforested areas, and that floodwaters deeper than 0.98 (95% C.I. 0.45–1.53) m do not accumulate suspended sediment or mercury. At four sites, floodplain accumulation of 0.8–1.2 m occurred over a period of 39 (95% C.I. 22–56) years, while sedimentation is negligible (mean: 0.1 m, median: 0.03 m) at other sites.

How Hydrologic Connectivity Regulates Water Quality in River Corridors

U.S. Geological SurveyUnited States Geological Survey; National Science Foundation Hydrologic Sciences ProgramNational Science Foundation (NSF)NSF - Directorate for Geosciences (GEO); USGS National Water Quality Program; DOE Office of Biological and Environmental Research (BER) in the Subsurface Biogeochemistry Program (SBR) as part of SBRs Scientific Focus Area at the Pacific Northwest National Laboratory (PNNL)

Geomorphology: A Canadian Perspective

Geomorphology is a hot discipline. Recent interest in river restoration, climate change, geomorphic hazards such as landslides and tsunamis, controlled floods, and other issues has increased the visibility of geomorphology as a profession. New methods involving the Global Positioning System, remote sensing, numerical simulation, laboratory experimentation, and novel dating techniques have created new research opportunities. The number of jobs in academia, industry, and the public sector is rising. What is the best way to convey this excitement to students, while at the same time properly training them? The traditional approach is an introductory course at the undergraduate level, built around a general textbook. When I teach geomorphology, I do not use a textbook but rather rely on original readings and field-based exercises to introduce students to geomorphic concepts and methods.