Geology Reference
In-Depth Information
9
FLUVIAL LANDSCAPES
Running water wears away molehills and mountains, and builds fans, floodplains, and deltas. This chapter covers:
running water
water-carved landforms
water-constructed landforms
fluvial landscapes and humans
Running water in action: floods
Plum Creek flows northwards over a sand bed between Colorado Springs and Denver in the USA, and eventually
joins the South Platte River. On 16 June 1965, a series of intense convective cells in the region climaxed in an
intense storm, with 360 mm of rain falling in four hours, and a flood (Osterkamp and Costa 1987). The flood had
a recurrence interval of between 900 and 1,600 years and a peak discharge of 4,360 m
3
/s, which was fifteen times
higher than the 50-year flood. It destroyed the gauging station at Louviers and swept through Denver causing severe
damage. The flow at Louviers is estimated to have gone from less than 5 m
3
/s to 4,360 m
3
/s in about 40 minutes.
At peak flow, the water across the valley averaged from 2.4 to 2.9 m deep, and in places was 5.8 m deep. The deeper
sections flowed at around 5.4 m/s. The flood had far-reaching effects on the geomorphology and vegetation of the
valley floor. Rampant erosion and undercutting of banks led to bank failures and channel widening. The processes
were aided by debris snagged on trees and other obstructions, which caused them to topple and encourage sites of
rapid scouring. Along a 4.08-km study reach, the average channel width increased from 26 to 68 m. Just over half
the woody vegetation was destroyed. Following a heavy spring runoff in 1973, the channel increased to 115 m and
increased its degree of braiding.
