Effects of Floodplain and Terrace Pit Mining

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Effects of Floodplain and Terrace Pit Mining

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Effects of Floodplain and Terrace Pit Mining

Effects of Floodplain and Terrace Pit Mining

Wet pit mining on floodplains (and terraces) involves conversion of land uses (typically riparian forest or agriculture) during and after the mining operation, channelization of the free-flowing river by levees and bank protection between pits and the river channel, and creation of warmwater lentic habitats that support non- indigenous fish species. So long as off- channel pits remain “isolated” from the free-flowing river, the principal effects on the channel are reduced channel migration and channel- floodplain interactions, physical habitat changes due to hydraulic effect of channelization, lack of natural channel banks and riparian vegetation and associated habitat along levees/bank protection works, trapping salmonids in pits and releasing non- indigenous fish (potential predators on salmonids) into the free-flowing river while the pit is hydraulically connected to the channel during floods, and changes in groundwater-surface water interactions potentially affecting temperature, water quality, and benthic invertebrate habitat and productivity. During excavation, if floodplain pits are kept dry by pumping, they lower local water tables, potentially dewatering nearby tributary channels and desiccating riparian vegetation and floodplain wetlands. When off-channel pits (inevitably) become captured by the channel, other impacts are introduced, including mixing non-indigenous predatory fish with salmonids, initiating bed erosion upstream by regressive knickpoint erosion and downstream by trapping bedload in the pit, and changing water temperature by mixing lotic with lentic waters.

Conversion of Existing Floodplain Habitat and other Land Uses

Effects of Floodplain and Terrace Pit Mining

In Washington state, floodplain and terrace gravel pits typically each cover about 4 hectares (10 acres) of land, which in most cases supported riparian forest or agricultural land use prior to mine development. Displacement of these uses is a direct effect of floodplain pit excavation and mine site development. Where an aggregate pit lies entirely above the water table (a dry pit), it is possible to reclaim the pit to agriculture or housing, similar to other quarries or open-pit mines.

However, floodplain pits typically intersect the water table for at least part of the year (wet pits), resulting in land-use conversion from farmland or riparian habitat to open-water pond. The scale of the landscape transformation effected by this pit mining is not widely appreciated, but becomes apparent by flying over river floodplains in light aircraft (Figure 31).

Collins (1995) documented a total of 152 floodplain pits along the Yakima River (counting those greater than 1.2 ha in size and deeper than the groundwater table). These pits, about 1 per kilometer of river, covered a total of about 580 ha (2,150 acres) of the Yakima River floodplain.

Historical aerial photographs of the Yakima River show numerous meander scars and oxbow lakes indicating active channel migration over a wide floodplain area. Floodplain gravel pits, in addition to highway prisms and railroad grades and urbanization, reduce floodplain connectivity and restrict channel migration, which, along with reduced base flows from irrigation diversions, have substantially reduced habitat diversity (Eitemiller 1999). Stanford (1998) hypothesized that reduced connectivity between channel habitats and shallow back water habitats created by periodic flooding and upwelling groundwater is one of the key limiting factor for salmonid populations.

Channelization/Levee Effects

Floodplain pits are commonly excavated close to the currently active channel, where the bestsorted gravels are typically found. To maximize the floodplain area accessib le for mining and to prevent the channel from eroding into the pits, the channel is commonly straightened and its banks stabilized with riprap. To prevent floodwaters from entering (and potentially destabilizing) the pits, levees are commonly constructed between the now-confined active channel and the pits. Thus, floodplain pits are typically accompanied by channelization.

Channelization has a host of negative impacts on river form and ecology (Brookes 1988). Those particularly relevant to salmonids include channel constriction, increased flow velocity and shear stresses and resulting reduced channel complexity, loss of high flow refuges, loss of riparian cover, and blow-out of channel gravels. Decreased surface area of pools and increased surface area of riffles have been documented as a consequence of channelization (Moyle 1976, Cederholm and Koski 1977). On the Tuolumne River, riffle slopes in channelized reaches between floodplain gravel pits are observed to be steeper than riffle slopes elsewhere, potentially driving riffles outside the range of acceptable depth and velocity conditions for salmonid spawning (Scott McBain, personal communication 2000).

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