ABSTRACT
On June 16, 1965, areas south of Denver, Colorado received up to 360 mm of precipitation in four hours, resulting in catastrophic flooding along Plum Creek, a sand-bed stream tributary to the South Platte River. Based on mixed-population flood-frequency analysis, the flood had a recurrence interval between 900 and 1,600 years; the peak discharge of 4,360 m3s−1 was 15 times that of the 50-year flood, the highest ratio ever recorded at a U.S. gaging station. Bottomland damage was extreme, and most geomorphic features and vegetation were extensively modified or destroyed. Along a 4.08-km study reach, average channel width of Plum Creek increased from 26 to 68 m; destruction of vegetation by the flood, followed by heavy spring runoff in 1973, caused increased braiding and a channel averaging 115 m in width. Lack of erosive discharges since 1973 has led to decreasing channel width.
Hydrologic and botanical field investigations suggest that channel narrowing and flood-plain reconstruction along Plum Creek has occurred in two principal manners: (1) On channel bars new vegetation reduced velocities during inundation and caused deposition of sand and relatively rapid aggradation of the recently revegetated surface. (2) Channel islands, which form downstream from channel obstructions, have developed stabilizing woody vegetation that also favors sand deposition and promotes enlargement. Channel islands grow allometrically in the downstream direction by annual increments; they induce channel narrowing by coalescing with other islands and flood-plain edges.
