FLOOD09060

• County: Statewide
• Community: Statewide
• Basin: Statewide
• Title: Statewide River Rehabilitation and Flood Plain Management Needs Inventory
• Date: 2/18/1998
• Prepared For: State of Colorado
• Prepared By: McLaughlin Water Engineers, Ltd.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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CONSTRAINTS

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There are several types of natural constraints on stream form including: geologic,
topographic, sedimentologic, and botanical. The degree to which a constraint affects a stream
type depends on the relative power of the stream, and the time scale involved. In the short-term,
there are many constraints on stream form, while on a geologic time scale stream erosion and
sedimentation eventually surpasses nearly every control. The time frame pertinent to river
management is 50 to 100 years which is short on a geologic time scale. Many controls are
affective for less powerful streams and diminish in importance as stream power increases.

GEOLOGIC

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Geologic controls can occur in two types: incisement into bedrock, or control of channel
grade by a protrusion of bedrock across the valley. Bedrock channels are typically found in steep
valleys where sediment transport significantly exceeds sources of sediment. Geologic grade
controls can be found in nearly any type of terrain and can often be identified by other bedrock
outcrops in the valley floor, The influence of geologic controls varies with the durability of the
rock formation ranging from hard granites to more erodible sandstone and shale.

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TOPOGRAPHIC

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Narrow, well defIned valleys are capable of confining stream flow to within the limits of the
valley walls. For less powerful streams, confinement may occur due to smaller features that are
the remnants of large flood, debris flow, or land slide events. The more powerful the stream the
deeper the valley that is needed to confine the stream.

SEDIMENTOLOGIC

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Streams may be confined to boulder bed channels that are the remnants of post-glacial wash
outs or catasrrophic flood events. The stream occurs within stable boulder beds that have a
threshold for movement that exceeds the present flood hydrology. This constraint is similar to
bed rock incisement of the stream. In a related case, a cobble bed channel may overfit the larger
boulder-bed channel.

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Stream sediments may become exhausted in denuded watershed. That shows as altered grade
and sediment size in a reach that is uncharacteristic for steeper terrain. This is most common in
mountain streams where the geology of the watershed is vety resistant to erosion or catastrophic
erosion has taken place.

BOTANICAL

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Large vegetation can locally effect the channel alignment and influence stream pattern. In
particular, large trees either as a mass of debris, or as individual spars or root wads can redirect
flow in bends. The regularity of stream meanders varies with relative stream power and
vegetative debris size. For low stream power, vegetation may dominate the channel section and
bank, increasing the stability of the bed and banks over the stability of the perimeter soils.

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Channels with vegetated banks tend to be deeper and narrower. An effect that is similar to
an increase in the silt-clay content of the channel perimeter soils.

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