Geoscience Reference
In-Depth Information
Case Study 3.2 Fluvial responses of rivers to inputs of mine wastes: active transformation and
passive dispersal
The response of rivers to inputs of both contaminated and uncontaminated particles can be viewed
as a continuum, with
active transformation
at one end, and
passive dispersal
at the other. In
active
transformation
, the massive input of large quantities of contaminated sediment engulfs the fluvial
system, causing changes in the types, rates and/or magnitudes of fluvial erosional and depositional
processes. This in turn causes the rivers to undergo significant transformations in channel form,
which influences the deposition and storage of the contaminated sediment. The classic study of
Gilbert (1917) demonstrated that river systems in the Sierra Nevada area, California, responded
to enormous inputs of metal mining waste by aggrading their channel beds by 3 -5 m during a
10 -20 year period after hydraulic gold mining ceased in 1884 (Case Fig. 3.2a(i)). After all of the
mining-contaminated sediment had been exhausted, the channel beds were slowly reworked and
degraded (over tens to hundreds of years), eventually returning to their pre-mining elevations
(Case Fig. 3.2a(i)). Gilbert (1917) modelled the downstream aggradation-degradation process
as a simple, symmetrical sediment debris wave. Graves & Eliab (1977) extended the work of
Gilbert (1917) and showed that the wave in fact was skewed (Case Fig. 3.2a(ii)); this reflected
the temporary floodplain storage of mine waste and its subsequent remobilization.
Active transformation
can also result in a channel metamorphosis from meandering to braided
in response to the large sediment input, followed by incision and reversion to a single channel after
mining (and the sediment supply) has ceased. This is well-illustrated by the River Nent, a tributary
of the River Tyne in north-east England, which was affected by large inputs of lead-, zinc- and
cadmium-bearing mining waste in the eighteenth and nineteenth centuries AD. Here, the single
channel and floodplain before 1820 was transformed into a broad, aggrading valley floor due to
the large influx of fine-grained, metal-rich sediment. After mining ceased in the early twentieth
century, lateral reworking was initiated, and incision of this floodplain began sometime between
1948 and 1976, eventually exposing bottom gravels by 1984 (Case Figure 3.2b).
By contrast, in
passive dispersal
, the channel and floodplain are not disrupted by the influx of
mine waste, because that waste is carried as part of the 'normal' sediment load. This is regarded
as a system at equilibrium. 'Natural' and contaminated sediments are transported together,
deposited in, and remobilized from, lateral and vertical (overbank) accretionary deposits, with
little physical impact on the system.
Even though the terms
active transformation
and
passive dispersal
were defined for mining-
affected river systems, they can equally be applied to rivers that are affected by other sediment-
borne contaminants such as nutrients, organics and radionuclides.
Relevant reading
Gilbert, G.K. (1917)
Hydraulic Mining Debris in the Sierra Nevada
. Professional Paper 105, US Geological
Survey,Washington.
Graves, W. & Eliab, P. (1977)
Sediment Study: Alternative Delta Water Facilities - Peripheral Canal Plan
.
Central District, California Department of Water Resources, Sacramento.
Lewin, J. & Macklin, M.G. (1987) Metal mining and floodplain sedimentation in Britain. In:
International
Geomorphology 1986
, Part 1 (Ed. V. Gardiner), pp. 1009-27. Wiley, Chichester.
Lewin, J., Bradley, S.B. & Macklin, M.G. (1983) Historical valley alluviation in mid-Wales.
Geological Journal
18
, 331-50.
Miller, J.R. (1997) The role of fluvial geomorphic processes in the dispersal of heavy metals from mine sites.
Journal of Geochemical Exploration
58
, 101-18.
