Channelization and Channel Modification Activities and Impacts
4.2.5.2 Environmental Effects
Levees act as a barrier for overbank flows. On un-leveed streams, flows periodically flow onto
the floodplain depositing sediment, flushing riparian aquatic environments, and generally providing valuable
habitat for aquatic organisms and waterfowl. The flora and fauna are adapted to periodic flooding and the
unique environment that it creates. Confining stream flows within a levee system creates a dryer
environment on the landside of the levee system and a wetter environment on the stream side. The dryer
environment results in changes in both flora and fauna that occupy the floodplain. Studies indicate that after
a levee systems are constructed, upland trees and vegetation colonize the floodplain. The lands between
the levee and the stream bank will experience more prolonged flooding with more extreme fluctuations in
water level. This may inhibit the growth of ground cover, thus reducing the available habitat for ground-
dwelling mammals (Fredrickson, 1979). For economical considerations, material used to construct the
levees generally are excavated from areas within the floodplain, resulting in vegetation removal and loss of
the habitat. The flat slopes used for levees in rural areas require large land requirements for the
embankments and berms.
4.2.5.3 Remedial Practices
To offset changes in riparian habitat, consideration is being given to the habitat provided by the
levees themselves and the adjacent borrow pits. Traditionally, the vegetation on levees is kept to a
minimum. Management of vegetation on levees was investigated on a project along the Sacramento River
(Davis et al., 1967). The results of the study indicated that with proper maintenance, certain species of
shrubs and plants could be allowed to grow without affecting the integrity of the levee. Additionally, the
study showed that the cost of maintaining vegetation on the levee was roughly twice the cost of traditional
levee maintenance (no vegetation), and that vegetation on levees provides the habitat for burrowing
animals that must be controlled. Borrow pits remaining from levee construction can serve as valuable
aquatic habitat. Normally, the pits will fill with rain water or groundwater after construction. Riverside
borrow pits will exchange water with the river system, thus recharging the pit with fish and other aquatic
organisms. Thus borrow pits partially compensate for the loss of aquatic habitat in the floodplain.
Additionally, siting levees further from the channel will conserve wetland environments between the levee
and the river.
4.2.5.4 Operation and Maintenance of Levees
Levees must be periodically inspected and maintained to provide the designed degree of flood
protection. Conditions affecting the integrity of the levee include erosion of the banks, seepage, and
damage from burrowing animals. Vegetation planted on the levees for aesthetic reasons should be well
maintained. Other vegetation that may affect the integrity of the levee should be removed.
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