| OCR Text |
Show Record 1867 center of the stream. Work of this character is done all over the country by the Coast Survey, and the bench marks indicated on the exhibit are established practically every three or four 1868 miles. Each of the contours which appear on this map along the 1871 show a rise, I thinks, of twenty- five or twenty feet. From these profiles one is able to determine from the slope in what sections 1872 of the river rapids occur. Topographers take elevations above and below each rapid with an instrument. At the various 1873 stages of the river a survey will show a different profile. As the river rises usually a number of rapids are obliterated on account of the height of the water. The effect of rocks or sand bars in the stream depends entirely on the stage of water. I 1874 would rather go down in high water than in low water, but wouldn't like to have it too high. I think river boatmen consider it most dangerous to get in between the rocks; if they had a chute right through a rapid, I do not think they would consider it difficult. I think a stream would have considerable rapids in it with a drop of ten feet to the mile; but if such a stream was unobstructed by rocks or sand bars, I do not think there would be any particular difficulty in getting down in a boat unless there were some awful curves that would throw you up against 1875 the bank. The chief dangers and difficulties arise from a com-bination of a drop in velocity with obstructions in the river bed. 1876 I do not think there were very many sand bars on this river. Sand bars may be indicated in places on the map. At mile 125 on the map you show me a sand bar is indicated, but that is not submerged at the present time. On page 4 of sheet D, at mile 44, a 1877 sand bar is indicated also at about Mile 45. Only those bars that are exposed when the topographer goes through are shown on 1879 the map. We show all features on the map which appear to the eye up to a contour line of 3900 feet. I don't think rocks are shown on the map. A topographical map is not intended for purposes of navigation at all |
