WATER STAGE REGISTERS 79 



in the mountain* and near the source the stage generally 

 ill.- <ia\ .nil decreases at night; at a station 

 fart) i the source or mountainous section the time of 



increase ot decrease in stage varies according to the distance. 

 Reservoirs storing stream water for use as occasion demand*, 

 'iiii i irrigation, or both, produce artil 



conditions that may cause sudden changes in stage. To obtain 

 .in a.vuralr ivon-il ..I , han^-- in M.i-r irr.--ul.ir in mt.-r.aU 

 and mi. .(ii.il in magnitude, and aUo of the quantity of water 

 n-h-ased from storage reservoirs as well as of the flow not 



Mor.-.l. .in aulomah. r.-ui-t.-r i- .-^rntial. 



\\IIKHK KECOROS ARE DESIRED ON A FLOOD-WATER 



MHI-:\M \\IIK II I- I>IC\ MOM OK INI M \\(. 



Krronl- of ilo\% o| a fliMxl-tt atT >ln\im \\hoM- rkilllifl l- 



dry most of the year are very <litii<-ult to obtain. Streams of 

 tin- type carry water at times of melting snow or of heavy rain- 

 fall. The rain may conn- in t)u form of a cloud-burst and the 

 flood- re-lilting an- of >hort duration, prrh.ip- 1 ~> or 2M minut--. 

 Even if a staff gage and a gage reader were available, the flood 

 would pass the gage before the reader arrived there. The 

 duration "! th<- flood depends largely on the size and char. 

 of the drainage area and the distribution of the rain fall. An 

 automatic register is necessary if a true hydrograph of the 

 stream is to l< <.i>t.iin<><]. Tin- float of such a register is usually 

 carried up \\ ith irn-at force and rapidity when the wall of water 

 or bore of tin- flood, sorn. -tim.- 1 ."> or 20 feet high, reaches the 

 register. Mm h foresight must be used to prevent damage to 

 th- automatic regist. i ti-ni such sources. 



4. WHERE COMPLETE RECORDS ARE DESIRED ON A STREAM 



u ML II FLOWS CONTINUOUSLY BUT IS SUBJECT TO SUDDEN FLOODS. 



The flow of a perennial stream that is not subject to floods 

 can be accurat. , m ,-,| |,\ n -a. lings from a staff or chain 



JM^I-: hut a stream suhjc* t t<> -ul.l-n floods can not beaccuratrU 

 gaged without automatic instruments. Not uncommonly the 

 gage reader at a station where a staff or chain gage is used 

 makes two readings .l.ul\ morning and evening. If a heavy 

 ram occurs h-t \\.-.-n the time of these two readings and causes 

 a sudden flood it i- more than probable that the morning and 

 al't. rnoon gage hn-lit- will give no inli< .iti.<n of the chanp<- in 

 stagr. I mt \\ ill -imply show a con-taut discharge. Since a large 



