SOIL SURVEY

-16-The majority of the lakes and streams are aesthetically pleasing bodies ofcold, clean mountain waters . Many of the streams and some of the lakes have coars eand highly permeable soils adjacent to them . If such soils are mismanaged throug himproper or over-use, then eutrification or siltation may occur and the clean attractivenessof these waters could be lost .CLIMATEClimate in Yoho National Park is determined by its position immediatel ywest of the Continental Divide where conditions are moderated by the Maritim eclimate from the west Pacific coast (Figure 9) .The changeability of the weather is a very important climatic characteristi cand is largely determined by the frequency and type of air masses affecting the area .In this portion of the Canadian Rockies, weather is determined mainly by two ai rmasses :(a) Polar Continental air masses which originate in the Yukon and Alaska .These air masses are cold and dry in winter and warm and dry in summer . Since the ycommonly move southeastward, parallel to the trend of the major valleys, thei rmovement through the Rockies is comparatively unrestricted . During the winter thes ePolar Continental air masses may spill westward over the top or through the larger gap sof the mountain system and bring cold temperatures as far as the west coast (Chapma n1952; Heusser 1956) .(b) The Polar Maritime air masses which originate in the Pacific . As this moistwarm air mass moves eastward across the mountain ranges, it becomes progressivel ycooler and dryer, so that east of the Rockies it may take on characteristics of Continenta lair masses . These masses follow one another from September to June, bringing precipitationto the mountains, particularly to the west facing slopes . In summer, the Pola rMaritime air provides less precipitation to Yoho than in the winter . As the masse sprogress eastward the shallow surface layer warms up and after mixing with dry ai raloft it becomes indistinguishable from dry Continental air east of the Rockies (Chapma n1952; Heusser 1956) .