FLOOD00235 - Laserfiche WebLink

Home Browse Search

19 Snow Pack The accumulation of snow within watersneds is lenerally refel:red to as snow pack. Where substantial snow pack develops, vster may be rel:a1ned as surface storage both in the frozen phase and as free water held in the pore spaces (Anderson et al. 1976) for considerable months b~fore passing through the watershed into the stream syste11\. Volua. and duration of detention is directly related to snow depth, air temperature, port' size, and initial free water content. Whereas snow pack may last severlll months at high elevations in the Rocky and other mountain ran,ges, it tends to be far more fleeting at lower elevations and especially i'n areas of high rainfall such as the Cascade Mountain range. In either case, a significant warming trend coupled with a prolonged heavy rainfall can cause r"pid snowmelt, heavy runoff, and significant flo,'ding. Several factors may operate singly or :Ln combinat.1on to delay snowmel t runoff and flOOding. Vegetation shading call significsll1tly maintai,n surface water storage by delaying snowmelt. Although snow und,er the forest canopy melts later and persists longer than snow in the open, it may melt more rapidly once melting begins, because meltinll begins later when tea'peratures may be much higher. In mixed vegetated and open areas snow melting tends to occur at different times and the total quantity rel,eased at anyone time is reduced. Likewise, topography also desynchron:Lzes snow melt. South feeing slopes always tend to lose snow pack soon,er than north facing slopes. Stream Flow Rivers, Streams. Creeks, and Brooks Some confusion has been generated in the scientif:lc literature by utilizing several words, all of which apply to channeHzed flow, and, indeed, may apply to channels without water. For example, Websters New World Dictionary (1970) defines "stream" as a flow of \tater running along the surface of the earth: specifically, a small "river." "River" is defined as a natural stream of water larger than a "crl!ek" and emptying into an ocean, lake, or another rivsr. "CrE'ek" is defjlned as a small stream, somewhat larger than a "brook." "Brook" is defined as a small stream, usually not so large as a river. Melst geologi!lts use the terms stream and river interchangeably and synon)'1llously to dE!Scribe running water of any size. However, some writings restrict the word river to identify the mainstream or largest tributary of a drainage netwclrk (Mor1sawa 1968). In order to eliminate confusion and the nece,ssity of sE'tting bo'Undaries that the words Jltl;,eam, river, creek, and brook imply, I: have chosen to favor the word -.tre..- and apply its usage to all SiZE~ of flowing water channels and especially to the entire longitudinally Unlted - drainage system extending from the most meager headwater beginnings to an arbitrarily identified end, mouth, or estuary. Thus no attempt is made in what follows to add to the confusion by distinguishing between varie.us sizes of flowing waters within channels by using the four terlllS intercheingeably.