ABSTRACT

A study was made to determine the character of deposits of a major flood along Bijou Creek, Colorado. These deposits consist of sand that was laid down in mid-June, 1965 during rapid movement of the upper flow regime. They are composed largely of horizontal or nearly horizontal layers that cover the channel and extend outward beyond the banks on each side of the stream commonly for distances of one-quarter to one-half mile or more. Typical thickness of these deposits ranges from two to three feet, but a maximum of 12 feet was measured at one place on West Bijou Creek.

Estimates indicate that horizontal strata constitute 90 to 95 percent of all deposits. The second most common structure of the flood plains is a tabular planar type of crossbedding formed of foresets developed mostly along outer margins of depositional sheets. Apparently such structures formed where water that had lost some of its force deposited sand along a sloping sediment front. Other locally developed structures include climbing ripple laminae and convolute bedding, which formed during waning stages of deposition and which apparently are characteristic of relatively sheltered areas away from the path of the main flood.

The dominant grade size represented by sand of the Bijou Creek flood deposits, based on mechanical analyses of 137 samples from the four localities studied, is medium grained (0.5-0.25 mm); 51 samples were in this class; 37 were dominantly fine grained; and 36 dominantly coarse grained. The very large objects, such as beams and concrete slabs from bridges, that were deposited during the storm indicate a velocity of water well above that necessary to carry fine to coarse sand; grain size in the deposits probably was determined by the size of sand particles available in the source area a few miles to the south. Sorting was fair in most of the 137 samples; it was good in only seven and poor in ten.

Data on the structure, texture, and shape of the sand bodies deposited by floods of major proportions should be useful in interpreting ancient deposits formed in a comparable environment. Also, they should help in relating the types of structures in a recognizable flood deposit to the specific stage of flood deposition, or stream regime, and in distinguishing the deposits of a river channel from those of adjacent flood plains.