Land Reclamation Program

Attachment D

Research Studies on the Impacts of In-stream Sand and Gravel Mining

Brown, Arthur and Lyttle, Madeline. 1994. Impacts of Gravel mining on Stream Ecosystems. University of Arkansas.
"Substrate degradation (erosion) that results from gravel mining disturbances causes several problems in addition to altering channel morphology and undercutting riparian trees. Fine sediments are released from intersticial spaces where they have settled, thereby increasing turbidity of the water. Benthic invertebrates, fish larvae, and fish eggs may also be entrained in catastrophic drift (Waters 1962) downstream…Aggradation buries benthic organisms (algae, invertebrates, fish eggs and larvae) and macrophytes under the transported sediments…
"Environmental degradation extended far beyond the boundaries of the gravel mining areas for several reasons. Headcutting has major consequences for many kilometers upstream from the mines. Downstream areas have too little gravel bedload to maintain normal stream channel structure because gravel is intercepted by flow alteration at the mines and then by mining removal. Silt travels long distances downstream as a plume of turbidity while gravel is being removed. During floods turbidity is higher than normal for even longer distances downstream due to the increased entrainment of sediments as a result of channel deformation."

Brown, Kenneth and Curole, Jason. 1993. Longitudinal changes in the mussels of the Amite River: Endangered species, effects of gravel mining and shell morphology. Louisiana State University.
"This study thus vividly confirms the earlier report by Hartfield (1989) that the mussel assemblage of the middle reach of the Amite has declined because of gravel-mining related activities. Gravel-mining results in bank erosion and a broader, shallower channel with a meandering flow (Hartfield, 1989), and we observed repeated instances of mussel stranded in shallow meanders and pools. The mussels apparently select the slower-flow, soft-sediment environments of these pools, but the pools are very susceptible to drying, as the river level drops. Gravel mining has evidently eradicated mussel assemblages in a major portion of the river, and could have even more drastic effects if mines expand further into the lower reaches of the river."

Roell, Michael. 1999. Sand and gravel mining in Missouri stream systems: aquatic resource effects and management alternatives. Missouri Department of Conservation.
"The stability of sand-bed and gravel-bed streams depends on a delicate balance among stream flow, sediment supply from the watershed, and stream channel form. Mining disrupts sediment supply and channel form, which can result in a deepening of the channel (incision) over great distances upstream and downstream of the mine site as well as sedimentation of habitats downstream. Channel incision often leads to accelerated bank erosion, a wider and shallower channel, and lowering of the floodplain water table. Channel instability and sedimentation from instream mining also can damage public infrastructure (bridges, pipelines, and utility lines) and result in losses of fishery productivity, biodiversity, recreational potential, streamside land and real estate value…"

Collins, Brian and Dunne, Thomas. 1990. Fluvial Geomorphology and river-gravel mining: a guide for planners, case studies included. California Department of Conservation, Division of Mines and Geology.

SUMMARY

Gravel is extracted from many rivers for use in industry or as part of flood-control programs.  In few cases is extraction carried out according to a plan which includes as assessment of possible effects on rivers.  This report is intended to guide planners who may be undertakinga program to assess, predict, or manage effects of gravel extraction.  The publication is organized into four parts, as summarized below.

Effects on Rivers of Gravel Extraction (p. 1-2).

The effects of gravel extraction on river morphology and sediment transport are summarized from published field studies and from analysis of several additional rivers on which we have supplemented unpublished studies.

The summarized effects include:

  1. Extraction of bed material in excess of replenishment by transport from upstream causes the bed to lower (degrade) upstream and downstream of the site of removal.

  2. Bed degradation can undermine bridge supports, pipe lines, or other structures.

  3. Degradation may change the morphology of the riverbed, which constitutes one aspect of the aquatic habitat.

  4. Degradation can deplete the entire depth of gravelly bed material, exposing other substrates that my underlie the gravel, which could in turn affect the quality of aquatic habitat.

  5. If a floodplain aquifer drains to the stream, groundwater levels can be lowered as a result of bed degradation.

  6. Lowering of the water table can destroy riparian vegetation.

  7. Flooding is reduced as bed elevations and flood heights decrease, reducing hazard for human occupance of floodplains and the chance to damage to engineering works.

  8. The supply of overbank sediments to floodplains is reduced as flood heights decrease.

  9. Rapid bed degradation may induce bank collapse and erosion by increasing the heights of banks.

  10. In rivers in which sediments are accumulating on the bed (aggrading) in the undisturbed condition, gravel extraction can slow or stop aggradation, thereby maintaining the channel's capacity to convey flood waters.

  11. The reduction in size or height of bars can cause adjacent banks to erode more rapidly or to stabilize, depending on how much gravel is removed, the distribution of removal, and on the geometry of the particular bend.

  12. Removal of gravel from bars may cause downstream bars to erode if they subsequently receive less bed material than is carried downstream from them by fluvial transport.

Assessing Gravel Harvesting Effects (p. 2-6)

In order to characterize the supply of gravel to downstream reaches and to assess or predict the effects of gravel removal, it is necessary to understand how sediment is produced and transported and how it interacts with river-channel morphology.  This part summarizes the following influences on river channel geomorphology.

Processes of basin sediment production are reviewed (p. 2-3) because the location and manner in which sediment is contributed to rivers influences the amount and durability of gravel supplied to downstream reaches.  A combination of observation and measurement in the field and on aerial photographs may be used to identify, located, and quantify sediment sources, and to define how they change through time and as a result of changes in land use.

Processes by which sediment is transported along rivers and the controls on those processes are summarized (p. 3-4).  Most material within the bed of gravelly rivers is transported as bedload, which accounts for a relatively small proportion of the total load of most rivers.  Most material is typically transported during high flows which occur on only several days per year.

The location and behavior of the depositional reaches of rivers typically exploited for gravel are explained in light of the downstream reduction in channel gradient and transporting ability, and resulting gravel bar deposition (p. 4-5).  A combination of field sampling, field observations, and analysis or archival materials, such as aerial photographs, maps, and survey data, can be used to characterize the patters of sediment transport and deposition and of changes in channel morphology.  The same approach may be used to determine the effects of various land an channel uses on downstream patters of sediment transport and channel morphology.

Processes of bedload transport, gravel-bar accretion, floodplain formation, and channel migration at river bends are summarized (p. 5).  Methods for estimating bedload transport are reviewed.  The report stresses that each approach must be applied only under the conditions for which it is valid, that it is desirable whenever possible to use several different methods, and that it should almost always be possible to derive a usable estimate of transport in an given river (p. 6-7).  Equations for bedload transport can be useful if carefully employed, but the results should always be checked against some independent field evidence.