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<br /> <br />vi <br /> <br />GLOSSARY <br /> <br />Migmatite. An intimate mixture of granitelike igneous rock and a metamorphic host rock <br />(schist or gneiss) in which the mixture is on a small scale but is sufficiently coarse to be easily <br />recognized by eye (Stokes and Varnes, 1955. p. 92). Common in Big Thompson Canyon. <br />Mean sea level (msl). The average height of the surface of the sea for all stages of the tide over <br />a 19-year period. <br />Millibars. A pressure unit equivalent to 1,000 dynes per square centimeter, convenient for <br />reporting a tmospheric pressure. <br />:\1ixing ratio. The ratio of the mass of water vapor to the mass of dry air. <br />Peak discharge attenuation. The reduction in peak discharge of a stream along the direction <br />offIow. <br />Peak stage. The maximum height of a water surface above an established datum plane; same <br />as peak gage height. <br />Pegmatite. An igneous rock of deep-seated origin having a very coarse average grain size and <br />an interlocking texture, composed of predominant feldspar and quartz, subordinate mica. <br />and various accessory minerals. <br />Percent slope. The vertical rise in slope per horizontal distance expressed as a percentage. <br />Thus, a lO-percent slope rises 10 feet in a distance of 100 feet. <br />Point bar. One of a series of low, arcuate ridges of sand and gravel developed on the inside of a <br />growing meander by the slow addition of individual accretions accompanying migration of <br />the channel toward the outer bank (Gary and others, 1972, p. 552). <br />Precipitable water. The amount of water contained in an atmospheric column if all the vapor <br />between two levels (usually the surface and 500 mb) was condensed. <br />Radiosonde. A balloon-borne instrument package for measuring and transmitting meteorQ- <br />logical data. <br />Rawinsonde. Meteorological data-collection system including a radiosonde and reflectors for <br />measuring winds by radar. <br />~currence interval. As applied to flood events, recurrence interval is the average number of <br />years within which a given flood peak will be exceeded once. <br />Rockfall. Rock material that plummets, bounds, or rolls down a precipitous slope. Rapid to <br />extremely rapid movement. (See Varnes, 1958. fig. 6, p. 22.) <br />Runoff. That part of precipitation which appears in surface streams. <br />Schist. A visibly crystalline foliated metamorphic rock composed chiefly of platy mineral <br />grains, such as mica, oriented so that the rock tends to split into layers or slabs. Very com. <br />man in Big Thompson Canyon. <br />Sediment. Fragmental material that originated from weathering of rocks and is transported <br />by, suspended in, or deposited by water or is accumulated in beds by other natural agencies. <br /> <br />Sheetflood. A broad or nearly continuous cover of floodwater flowing sheetlike down a slope, <br />as opposed to water concentrated in rills or rivulets. A result of intense but short-duration <br />rainfall. <br />Sheetwash. Sediment picked up and redeposited by sheetflooding. <br />Slump. A landslide characterized by a shearing and rotary movement of a generally independ- <br />ent mass of rock or earth along a curved slip face (concave upward) and about an axis parallel <br />to the slope from which it descends. and by backward tilting of the mass with respect to that <br />slope so that the slump surface often exhibits a reversed slope facing uphilllGary and others, <br />1972, p, 6671- <br />Stage. The height of a water surface above an established datum plane. <br />Stage-discharge relation. The relation between gage height and the amount of water flowing <br />in a stream channel. <br />Stream competence. The measure of the ability of a stream to transport sediment. <br />Strike valley. A valley formed by differential erosion of alternate layers of relatively resistant <br />and nonresistant rocks, so that the valley coincides with the trend (strike) of the nonresistant <br />layers and the bounding ridges coincide with the resistant layers. <br />Talus. An accumulation of more or less angular rock fragments derived from and lying below <br />a steep slope or cliff. <br />Time of day. Expressed in 24.hour time. For example, 6:00 p.m. is expressed as 1800 hours <br />Mountain Daylight Time (MDT). <br />Totals index. Defined as 2(Ta5o-T,oG)-D~,o where T~5D is the 850 mb temperature, T~oo is the <br />500 rob temperature and DBlo is the 850 mb dewpoint depression, all expressed in oC. <br />Unit discharge. The average number of cubic feet per second flowing from each square mile of <br />area drained by a stream. assuming that the runoff is distributed uniformly in time and area. <br />