is about 400 cubic feet per second and sometimes less than 100 cubic feet per second, meanders in a narrow stream over the wide sandy bed, the balance of the bed being dry bars of sand and silt. During dry times various forms of vegetation spring up on these bars. If such dry times are shortly followed by high water, this vegetation is usually destroyed. If, however, the period of drought is prolonged, the growth sometimes becomes so large that high water cannot destroy it, and the building of an island or obstruction is started. Each successive flood usually adds to it and builds it up. The flood waters, carrying, as they do, great quantities of sand and silt, as they enter the growth covering these islands, have their velocity reduced and much of the material hitherto held in suspension is deposited on the island. In this way flood after flood deposits successive layers of silt. Islands and other obstructions are thus increased in height until they present such formidable obstacles to stream flow that the river is compelled to leave its course and flow through the more open portions of the valley.

CHAPTER II.

STREAM-FLOW MEASUREMENTS.

The department is now maintaining three gaging stations on the Arkansas river, one on the Pawnee river, and five on the five principal irrigation canals taking water from the Arkansas river. The stations on the Arkansas river are located at Syracuse, Garden City and Larned. The station on the Pawnee river is also at Larned. These stations are all equipped with automatic recorders which make continuous records of the gage heights of the water passing the gages.

The gages on the Arkansas at Garden City and Larned, together with the one on the Pawnee river, were installed coöperatively by the irrigation commissioner and the United States Geological Survey. The irrigation commissioner furnished the well and shelter, while the United States Geological Survey furnished the recording instrument for the gage at Garden City and the one on the Pawnee at Larned. The United States Geological Survey furnished the entire installation on the Arkansas river at Larned. Gages at the coöperative stations are being maintained by the Irrigation Department, and discharge measurements, for the most part, are made by this department.

The greatly increased interest in irrigation in the Pawnee valley, together with the gradually increasing acreage being brought under irrigation, has brought about the necessity for some information concerning the flow of water on that stream. Most of the irrigation projects on the Pawnee involve the construction of channel reservoirs for the storage of water. In this connection information concerning the flood flow of the stream is necessary for the design of dams which will pass such floods without flooding adjacent land. As the acreage under irrigation expands, more storage projects will be built. The number of such projects and their extent will of course depend on the amount of water available, and therefore presents the necessity for authentic information regarding the available amount of water on that stream.

The season of 1922-23 started in very dry and continued so until nearly the middle of May. Following this date the Arkansas river was subject to

a succession of very sharp, sudden floods, only one or two of which, however, reached the overflow stage. During the winter of 1922-'23 the river carried an unusual amount of water, which continued through the spring and up until almost the end of June. Following this the water supply dropped off very rapidly and the irrigation ditches suffered from a very deficient water supply during the whole months of July, August and September.

During the third week in March, 1924, an ice gorge was formed in the Arkansas river near Syracuse. With the breaking up of the gorge large quantities of ice were strewn all along the channel of the stream. This condition caused an abnormal flow to be recorded at Garden City and at Larned, due to the narrowing of the river channel by ice and the impounding of the water above the gorge. A part of the bridges at Hartland and at Kendall were torn away by the ice. The bridge at Deerfield was also somewhat damaged. The series of river floods in the summer of 1923 furnished an excellent opportunity to secure information of value in flood forecasting. Information concerning eight of the principal floods is shown in the following table, which gives the date and time of the arrival of the floods, the gage height reached and the maximum flow of each of the three river stations. To properly interpret the information in this table it is necessary to study it in connection with the stage of the river immediately preceding the flood period, and local rainfall.

ARKANSAS RIVER AT SYRACUSE.

Location. At highway bridge half a mile south of Syracuse, in section 18. township 24 south, range 40 west.

Drainage area. Approximately 25,500 square miles.

Records available. June 20, 1921, to September 30, 1924.

Gage. Gurley automatic graphic water-stage register on downstream side of bridge pier, and staff gage on outside of gage well.

Discharge measurements. Made from bridge during high water and by wading at low stages.

Channel. Shifting.

Extremes of discharge. For the two-year period ending September 30, 1924 Maximum stage, 6.35 feet, August 23, 1923 (discharge 18,000 second-feet); minimum stages, 0.93 feet, October 5 and 29, 1923 (discharge 4 second-feet). Ice. Stage-discharge relation apparently affected but little by ice excer: during March, 1924, when an ice gorge occurred.

Diversions. There are court decrees for the diversion of approximately 12,825 second-feet from the Arkansas and its tributaries above this station i Colorado, and two small diversions above it in Kansas.

Accuracy. Rating curve fairly well defined between stages of 0.94 and 52 feet. Daily discharge ascertained by applying hourly gage heights to rating Records fair.

curve.