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<br />6 <br /> <br />third order stream may have 9 first order tributaries (it has to have at <br />least 4) and 3 second order tributaries (it has to hav,a at least 2). The <br />main branch of the stream is always designated by the highest order number <br />of the drainage system or watershed. It often follows that first order and <br />some second order streams may tend to carry intermittent flows, especially <br />in the semiarid and arid parts of the country. As a p()int of refe:rence the <br />Mississippi River entering the Gulf of Mexic:o is classJlfied as a t..elfth <br />order stream which may be the maximum size (:lassification in North America. <br /> <br />( <br /> <br />The stream order method of analysis offers the advantage of grouping <br />streau according to relative size, length, numbers, and percent of the <br />total stream milage (Table 2). Stream orders 1, 2, and 3 range in width <br />from 0.5, 1-2, 3-6 meters, respectively, and are considered s:oall ntreams <br />(Cwaa1ns 1975). Orders 4, 5, and 6 are considered medj,um-sized streams and <br />are progressively wider. Orders 7 to 12 are consideredl large stre~'ms. The <br />significance of this type of grouping may be greatest .'hen one considers <br />the total number of miles in the conterminous U. S. (or the world) since it <br />has been shown that small streams (orders 1 to 3) comprise 85 percEOIlt, <br />mediu..-sized streams (orders 4 to 6) 12 percent and large streams (orders 7 <br />to 12) only 3 percent of the total stream milage (Leopold et al. 1964). <br />Like trees, the trunk and branches of stream systems are totally dominated <br />by small tributaries (branches) that join with medium-sized tributa.ries <br />(secondary branches) which cullllinate in the final "main stem" or trunk that <br />flows into lakes and estuaries (Table 3). <br /> <br />r,UL! 2 <br /> <br />Co.parl.oa of .tre. order Il'oupln&1 according I:u ulatJ.ve .blt 11'00. p~rcel1t <br />of total .tr..~ .ll..e In the United State_. <br /> <br /> RElAtIVE PERCENT OF <br /> ORDER stREAM TOTAl. <br /> NUM6ER $llE STR~AH <br /> till.AGE <br /> 1.2.3 ....11 " <br /> 4. 5. 6 Jokldlu. 12 <br /> 7 to 12 lAra. 1 <br />The Laws of Stream Ordering <br /> <br />Map studies and field verification utllhing United States Geological <br />Survey quadrangle mapping with a scale of 1/2.1. ,000 have been used to assign <br />a number to each stream segment according to the stream order rules., The <br />number of segments of each order are tallied to yield the figures in Table <br />3. Illese types of studies of the linear stre<lll\ sequence of 1IlAny watersheds <br />have conf1ped a number of relationships bet..een stream order and the: <br />I. bifurcation ratio, <br />2. number of ordered segments, <br />3. length of ordered segments, <br />.. mean basin area, <br />t5. mean slope or gradient. <br />