Rapid Downcutting Research Articles

River erosion and bank stabilization — North Saskatchewan River, Edmonton, Alberta

The postglacial geology of the North Saskatchewan River valley in Edmonton is reviewed. Using radiocarbon dating by others, limited land survey, and aerial photography measurement, it is inferred that the river valley was quickly formed after glaciation during a period of rapid downcutting. Over the last 8000 years, the rate of vertical downcutting has been about 25 times slower than it was during the initial 5000 years following deglaciation. As the river reached its base level, lateral river migration occurred and erosion against the low-lying river terraces has been substantial. It is inferred that over the past century the lateral migration is probably at the rate of at least 30 m/century.Low-level erosion protection berms are required to stabilize the toe of a variety of landslides; however, they do not completely eliminate the causes of upper slope movement. The engineering considerations underlying the construction of four protective works are discussed and overall costs are presented.

Early Pleistocene Submarine Canyon, Boso Peninsula, Japan: ABSTRACT

The Boso submarine canyon was one of the first fossil canyons to be described. It is also one of the best exposed, because careful quarrying of the economically useful gravel fill has exposed the noneconomic marly rocks into which the canyon was cut. These country rocks (Umegase Formation) are gently dipping, cream-colored siltstones and mudstones. In contrast, the canyon fill (Higashi-higasa Formation) consists of brown to yellow sandstones with marked lenses of polymodal and polymictic conglomerates (with small pebbles of granites, basalts, cherts, and basic tuffs from the Chichibu terrane and much larger clasts of marlstone up to 1 m in diameter). There are also some boulder beds and armored mud balls and many early Pleistocene shelly fossils. End_Page 551------------------------------ Within the canyon-fill sediments, there are some unusual water-escape structures. The southern wall of the canyon is well exposed in a large quarry, where four steps occur in a vertical height of about 60 m. Upcanyon, these pass into fewer but higher steps. Small overhangs caused by protruding bedding surfaces are original features, as gravel fill still adheres to the marly walls. Canyon downcutting toward the east is shown by widening of the present outcrop of the fill in that direction, and eastward-moving paleocurrents are indicated by boulder imbrication. Rapid downcutting and filling is suggested by the well-preserved wall overhangs, and channeling within the fill sediments suggests that the deeply cut canyon was filled by several successive influxes of sediment. End_of_Article - Last_Page 552------------

Preglacial (Teays) and Early Glacial Drainage in the Cincinnati Area, Ohio, Kentucky, and Indiana

The preglacial rivers of southwestern Ohio, northern Kentucky, and southeastern Indiana flowed toward the north and joined with the west-flowing trunk river, the Teays, in central Ohio. The main tributary valleys to the Teays River in this region—containing the Old Kentucky, Manchester, and Old Licking Rivers—were meandering and incised to a depth of 30 to 60 m (100 to 200 ft) below the upland level. The bedload carried by these rivers had a distinctly southern source. During the first glacial advance into central Ohio and Indiana in pre-Illinoian time, the Teays drainage was dammed, and impounded waters filled the valleys. East of Cincinnati in the Manchester and Old Licking River valleys, thick lacustrine clays were deposited. West of Cincinnati, however, in the Old Kentucky River basin, there is very little lacustrine sediment, suggesting that either (1) the Old Kentucky River was not ponded because its flow had already been reversed as a result of preglacial piracy by the west-flowing Old Ohio River, or (2) glacial ponding occurred but was short-lived because overflow from the Old Kentucky River basin west into the Old Ohio River basin caused rapid downcutting of the divide between these basins. Thick lacustrine sediment in the Manchester and Old Licking River valleys indicates a long period of water impoundment. Hence, deepening of the spillways between this lake and the Kentucky River basin to the west must have taken place very slowly. The lake probably existed for many years before the addition of water overflowing from the Teays tributaries in southeastern Ohio, West Virginia, and western Pennsylvania began to contribute to spillway erosion. Eventually, probably as a result of this initial glaciation, a new west-flowing Ohio River was established, approximately along its present course from Pittsburgh to southern Illinois. The new river closely followed the trend of the Teays-age valleys but seldom coincided with those old meandering valleys in the Cincinnati area. Entrenchment of the new Ohio River took place soon after the old valleys were abandoned, leaving many high-elevation preglacial valley remnants south of the pre-Illinoian glacial boundary. The initial post-Teays drainage pattern was modified in several places when Illinoian glaciation, and, possibly, a second pre-Illinoian advance (stade?) invaded the Ohio River valley north of Cincinnati and at several points to the east and west of the city.

RAPID EROSION OF THE RIVER URA, NAGANO PREFECTURE, AND ITS INFLUENCES DOWNSTREAM ON THE TRUNK RIVER

In 1911, there occurred a large landslide on the steep slopes of Mt. Hieda, Nagano prefecture, to which much attention was paid by geologists at the time. So much debris was produced that the river bed of the Ura, a tributary of the Hime River, was raised over 50 metres. In this paper, the author firstly clarifies the recent development of the mountain slopes and valleys, and then attempts to evaluate the importance of the severe erosion along the Ura River as a source of the bed load in the lower reaches of the Hime River. The results obtained are summarized as follows:1) The large landslide on Mt. Hieda occurred on the slope of this highly dissected strato-volcano, and is closely related to the heavy rain which fell four days before the occurrence, but is fundamentally due to the nature of the fragile rocks containing solfataric clay with abundant joints (Fig. 1, geological map).2) The landslide debris originally swept 6 kilometres down the valley and dammed the Hime River at its confluence with the Ura. The debris was deposited in the valley, elevating the valley floor a maximum of about 100 metres and steepening the gradient. The quantity of mudflow deposit is estimated roughly at 1.5×108 m3.3) The surface of the mudflow deposit is comparatively rugged with many flow mounds (Fig. 2). Just after the catastrophe, the superimposed river bed started incising itself due to the decrease in material provided from the slopes. The mudflow dammed the Hime River at the confluence, forming a lake. When this dam itself was washed away during the floods in the following year, a knick point was formed on the lower reaches of the Ura River, but this disappeared before it had time to migrate far upstream because of rapid downcutting in the upper reaches and consequent deposition along the lower reaches. Vertical erosion was rapid in earlier stages but lateral erosion became more important during later stages.4) The mass eroded during the past 50 years is calculated to be 2.4×107 m3. The Ura introduced such heavy loads into the Hime that the river bed of the latter was elevated a maximum of 30 metres and caused an increase in the gradient below the point of confluence due to the bed load which may well persist for a long time (Fig. 4).5) From the standpoint of the source of bed load, it can be considered that the materials of the Hime below the confluence were derived from the three sources; (a) Mt. Hieda and its mudflow deposits, (b) Mt. Kazafuki ((a) and (b) were transported by the Ura), (c) the upper area of the Hime River. The proportions of debris supplied by each river were found by calculation from the lithologic analysis of gravel as follows: (a) 41%, (b) 26%, (c) 33%. Area of the region (c) is twenty one times as large as the combined area of the regions (a) and (b), which indicates the high rate of erosion in the latter. Consequently, it was found that more than half of the gravels of the Hime River came from the Ura River dissecting the mudflow deposits of 1911, although the Ura is only a small tributary of the Hime.6) In conclusion, it was found that the erosion in the Ura River during the past 50 years has progressed enough to show a conspicuous topographic change in the bottom of the valley, and that the heavy load resulting from the rapid erosion has a strong influence on the stability of the Hime River.