FLOOD02337

• County: Statewide
• Community: State
• Stream Name: South Platte River
• Basin: South Platte
• Title: Geomorphic Assessment at Surveyed Cross-sections South Platte River
• Date: 6/1/1996
• Prepared For: UDFCD
• Prepared By: Michael Stevens
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Range of Change Number Higher Number Lower
ft in 1996 in 1996
0.1 to 0.5 15 7
0.6 to 1.0 4 4
Ll to 1.5 2 I
1.6 to 2.0 2
2.1 to 2.5 0 1
2.5 to 3.0 2 2
Total 25 16

AGGRADATION OR DEGRADATION

The thalweg is the lowest point in the river cross section. It is almost always, but not
necessarily, in the low-flow channel. Where the river is wide, bars of river-transported and
deposited sediment fonn on one or both sides of the low-flow channel. Commonly, a bar is
composed of the upstream bank material if that bank is on the same side of the river and is
eroding. If there are two low-flow channels in a cross section, the main and a side channel, the
bar between them is a middle bar. Otherwise, the bars are side bars, or alternate bars if there
is a sequence of bars on each side of a long straight reach of riveL Point bars are deposits on
the inside or convex side of bends. A middle bar that becomes vegetated and grows in height
to near floodplain level becomes an island

The bars can be high or low, bare or vegetated. Nonnally, the upstream end of the bar is lower
relative to the water surface than the downstream end. Bar sediment sizes grade from coarse to
fine in the downstream direction and from water's edge towards the bank. Willows can colonize
any part of the bar, but commonly grasses and weeds first appear at or near the water's edge. As
a result of the scouring effect of the prolonged high spring runoff in 1995, the bar surfaces are
mostly bare and clean. New snags on bars are rare, but old snags often remain. These snags are
usually old cottonwoods of stature.

The riverbed level is taken as the average level of the active channel. This includes the low bare
bars but not the high vegetated bars. In the Urban reach, the active channel width is the distance
between the base of the two river banks. By definition, the riverbed level is higher than the
thalweg level. The maximum bar height in a cross section is taken as the difference between the
level of highest point on the bar and the average bed level. The elevations of both the river bed
and the top of the bars at the monitored cross sections are given in Table I. Several cross
sections have not been surveyed lately, probably because the water depth was too great to be
waded.

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