486 PROCEEDINGS OF THE ALBANY MEETING 



RIVER ACTION PHENOMENA 

 BY JAMES E. TODD 



Contents 



Page 



Introduction 486 



Cutting down of river bottom in time of flood 486 



Mutual flood-relief channels 487 



Velocity checked by overflow, though slope is increased 488 



Iktroduction 



Altbongli the phenomena mentioned in this paper have been alluded to in the 

 writer's previous papers and to some extent by otliers, lie ventures to call more 

 direct attention to them. They have quite important practical as well as scientific 

 value and seem worthy of being more distinctly recorded than they have been. 



Cutting Down of River Bottom in Time of Flood 



The first is the fact that in time of flood the bottom of a river cuts down as the 

 surface rises. It has long been recognized that in floods the slope and velocity in- 

 crease, and therefore the conading and transporting power is increased, but the 

 extent to which this is done, especially in streams abounding in sediment, has not 

 been clearly set forth. 



We are fortunate in having distinct testimony on this point from experienced 

 engineers. Mr L. C. Cooiey, of the United States corps, states that at Nebraska 

 City, "when the river strikes quite sharply against a high blufi", though it ordi- 

 narily is but 40 or 50 feet in depth, when it reaches high water it is sometimes 90 

 feet" — that is, it picks up 20 or 25 feet of sand and gravel at the bottom as it rises 

 15 or 20 feet.* 



At the bridge near Blair, Nebraska, Mi E. Gerber, assistant engineer for the 

 Fremont, Elkhorn and Missouri Valley raihoad, watched very closely the cutting 

 of the river to see ho\v it might endanger the bridge which had been quite recently 

 erected. Not only was the gauge read regularly but soundings were taken about 

 every 10 days during the flood, from June 23 to August 18, 1883. It was found that 

 although at very low water the river was only a little over 10 feet deep, when it had 

 arisen 10 feet, it had cut down to bed rock, which was 50 feet below low-water 

 mark.f 



Mr George A. Lederle, engineer of the Omaha bridge, reports similar })henomena 

 at that point, but not so completely. 



At Blair, Omaha, Platsmouth, and Nebraska City the bed rock is 50, 90, 55, and 

 85 feet, respectively, below low-water mark, and in every case shows gravel, sand, 

 and clay boulders resting directly upon water-worn rock. Although no observa- 

 tions have been made when the water was cutting below 90 feet, there seems to be 

 no good reason for doubting that in beds and narrow places in the river the bed 

 rock may be sometimes touched below that level, so that in the shifting of the 

 stream the whole bed-rock bottom of the valley from bluff to bluff has been swept 



* Report of the United States Engineers, 1879-80, part ii, pages 106G-107I. 

 tU. S. Geol. Survey, Bulletin 158, p. 151. 



