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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The sediment on the bed of the South Platte River is small cobbles (2 to 5 inches in diameter)
in the steeper upstream reaches to cobbles, gravel, or sand in the lower reaches. But deposits
of sand, gravel, or cobbles can be found resting together in many places. In the Urban section,
often there are chunks of concrete rubble or riprap on the channel bed. In a few spots, the weak
rock of the Denver Formation outcrops on the river bed. There is such a spot between Cross
Sections 13 and 14 and another between Cross Sections 47 and 48. The Denver Formation
erodes very slowly so it controls riverbed degradation.

In the Rural segments, there is often a very small depression between a side bar and the river
bank, almost always dry at low flow. This depression appears to be influential in maintaining
a linear growth of woody vegetation along the base of the bank, mostly willows and
cottonwoods.

Degradation is the general lowering of the river bed in a long reach of river over time. In
contrast, aggradation is a general increase in the bed level. If the lowering is local, not over a
long reach, it is general scour. General scour occurs, for example, in the river under a bridge
with an opening less than that of the river upstream. The scour holes in the bed around bridge
piers are local scour.

To help identifY aggradation and degradation, the history of the thalweg level and average bed
level at each monitored cross section was studied (Table 3). If both the thalweg and the average
bed level changed in one direction in the period of record, then degradation or aggradation is
said to have occurred. The thalweg level can change but the average bed level does not. This
is not aggradation or degradation. Sometimes, the average bed level cycles up and down from
the mean without much net change during the period of record (Fig. 3). This cyclic behavior is
deemed stable, the same as if the bed did not change.

In all, the river bed is judged to be stable. The severe degradation experienced in the early
1980's (Stevens et a1. 1990) is no longer to be found. In 1996, 15 cross sections had degradation,
the most being 2.0 and 2.7 ft at Cross Sections 37 and 38, respectively (Fig. 4). Ten of these
experiencing lowering are in the Urban reach between Cross Sections 29 and 43, with much of
the lowering occurring in 1995, the apparent result of the long, high spring runoff.

Four cross sections experienced aggradation. Three of these are in the downstream Suburban
reach between 104th Avenue and Interstate 270. The other is Cross Section 9 in the Rural
Reach. The most aggradation was 2.1 ft at Cross Sections 9 and 28.

In June 1986, the river was flowing through a 22-ft deep gravel pit at Cross Section 23. In
November 1987, the pit had filled with river-transported sediments and has remained stable until
construction on channel modifications for water quality improvement in this reach began in
1995. The modifications were undertaken by the Metro Wastewater Reclamation District at an
estimated construction cost of$2,372,000. Aggradation immediately upstream at Cross Section
24 was the apparent result of the Metro project.

Construction ofthe Confluence Park river improvement has resulted, by design, in lowering the
river bed 2.0 and 2.7 ft upstream at Cross Sections 37 and 38.

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