Jerry R. Miller

The role of fluvial geomorphic processes in the dispersal of heavy metals from mine sites

Abstract It is not uncommon for more than 90% of the total metal load in rivers to be transported in the solid phase, either sorbed onto particle surfaces and coatings, or incorporated into mineral grains. Fluvial geomorphic processes are therefore of fundamental importance in the transport and fate of heavy metals derived from mine sites. In this paper, the role of physical processes in the dispersal of heavy metals in river systems are reviewed for channels that have (1) remained relatively unchanged in terms of process and form following the introduction of mine wastes, and (2) exhibited a significant metamorphosis in channel form in response to the influx of mining and milling debris. In general, all processes responsible for the variations in metal concentrations within sediments moving through stable channels also operate in channels undergoing metamorphosis. However, downstream, lateral, and vertical patterns in metal values tend to be more complex where channel transformations have occurred. This complexity results, in part, because temporal and spatial changes in the types, rates, and magnitudes of erosional and depositional processes lead to highly variable stratigraphic sequences of post-mining age, and because greater quantities of contaminated debris is stored along the channel margins where it can be eroded and sporadically redistributed during times of flood.

Effects of the 1997 Flood on the Transport and Storage of Sediment and Mercury within the Carson River Valley, West‐Central Nevada

Intense, warm rains falling on a heavy snowpack in the Sierra Nevada at the end of December 1996 produced some of the largest floods on record in west‐central Nevada. Within the Carson River basin, a peak discharge of 632 cm was recorded at the Fort Churchill gaging station on January 3, 1997, a flow exceeding the 100‐yr event. Geomorphic impacts of the event, and the redistribution of mercury (Hg) released to the Carson River valley by Comstock mining operations during the mid‐ to late‐1800s, were assessed by combining field data with the interpretation of aerial photographs. Geomorphic impacts included significant increases in channel width, measuring up to 280% of preflood conditions, and large‐scale shifts in channel position, ranging from <10 to 110 m. Both changes in channel width and position vary as a function of valley morphometry (width and slope) and differ from the long‐term trends measured from 1965 to 1991. The 1997 flood also produced widespread overbank deposits that vary morphologically and sedimentologically according to distance from the channel and the nature of the vegetation on the valley floor. Within the overbank deposits, Hg is primarily associated with the fine‐grained (<63 μm) sediment fraction, which makes up a larger percentage of the deposits immediately adjacent to the channel and at the extremities of overbank deposition. Mass balance calculations demonstrate that, along reaches with narrow valleys (<450 m), approximately 10%–65% of the sediment eroded from the channel banks was stored in overbank deposits, whereas more than 90% of the sediment eroded along reaches with wider valleys was stored on the valley floor. Locally, however, storage exceeded 650% where meander cutoff was extensive. The above data indicate that the erosion, redeposition, and storage of sediment and sediment‐bound Hg were greater along reaches characterized by low gradients and wide valley floors. Downstream trends in Hg concentration within the channel bed did not change following the 1997 flood and are presumably controlled by the overall structure of the system, including valley morphometry, the location of tributaries that deliver “clean” sediment to the channel, and the distribution of Hg within the valley fill.

Role of tree dams in the construction of pseudo-terraces and variable geomorphic response to floods in Little River valley, Virginia

Geomorphic response to a 1985 flood in Little River valley, northern Virginia, was different in magnitude and style from the largest historic flood in the same valley in 1949. The primary geomorphic activity during the 1985 flood was severe bank erosion and channel-gravel deposition rather than the debris flow and avalanching of the 1949 event. An unusual and widespread phenomenon of the recent flood was that large trees eroded and transported by the floodwater were aligned parallel to the river banks and, at isolated sites, were braced and stacked against trees still standing on the floodplain. Lateral barriers or dams created from these displaced trees allowed the channel to be locally aggraded above the level of the flood-plain. In these reaches, little, if any, river gravel was deposited on the floodplain, even though the adjacent channel floor was raised well above that surface. The river has now shifted around the filled segments, leaving flat, isolated surfaces, underlain by channel gravel, standing above the level of the modern floodplain. These features may be mistaken for terraces alter they become vegetated and the trees bracing the gravels decay. Interpreting these surfaces to be terrace remnants would lead to a faulty reconstruction of geomorphic history in the Little River valley and other valleys where floodplain morphology is controlled by infrequent flood events.

The disruption of Grassy Creek: implications concerning catastrophic events and thresholds

Two catastrophic events, occurring simultaneously in the valley of Little Grassy Creek, IL, allow for an examination of the threshold concept in geomorphology. Movement of debris associated with failure and sliding of valley-side material, caused damming and avulsion of Little Grassy Creek. Slope and river disruptions, both severe in character, were linked because the effect of one event (slope failure) was the cause of the second event (fluvial avulsion). The slope failure represents a true threshold-crossing event because the results are irreversible on a graded-time scale. In contrast, the fluvial disruption was not a threshold crossing, although the event was catastrophic and short-term instability occurred. In the fluvial case, a new channel developed, and the re-establishment of equilibrium, as estimated by channel characteristics, occurred within 10 years. The river system functions as it did before the slope failure/avulsion, though the channel reach is now in a different location. Criteria needed to employ thresholds to explain geomorphic events are suggested, and a definition of thresholds as time-dependent phenomena is presented as a means of reducing confusion over the use of the threshold concept.

Geomorphic response to minor cyclic climate changes, San Diego County, California

Abstract Short-term episodic cycles of wet and dry patterns of climate are common in southern California. Wet intervals, like the one in 1978-83, are often characterized by more than double the average annual precipitation. The impact of these episodic climatic fluctuations on landforms and surficial processes has not been well documented for areas inland of the coast. The response to these cycles may be significant in the evolution of hillslopes and fluvial landforms, and may have significant implications for geologic hazards in this rapidly developing region. Using aerial photographs and field investigations we found little response to the 1978–1983 wet interval on upland hillslopes, but documented significant response on alluvial fans and in channels in desert piedmont areas. These observations may lend support to the Langbein-Schumm (1958) model relating sediment yield to precipitation. A variety of techniques, including dendrogeomorphology, studies of the weathering of clasts, soil stratigraphy, and aerial photo mapping were used to discern at least six units on alluvial fans ranging from Late Pleistocene to present. Terraces along active fan channels and the San Felipe River record a geomorphic record of the most recent wet intervals (ca. 1940 and 1980) as a significant depositional event. Geomorphic responses to the wet interval along the San Felipe River were complex, varying locally according to controls on sediment storage and downstream transfer through a recently integrated drainage system. Additional complex responses to the wet period were experienced in selected sites where antecedence and response times may be measured in months or even years.