Obviously practical reasons can be found for certain tribal variations. In some areas, the low ends appear to ensue from the use of the canoe in open water, where the wind resistance of a high end would make paddling laborious. In others the low ends appear to result from the canoe being commonly employed in small streams where overhanging branches would obstruct passage. Portage conditions may likewise have been a factor; low ends would pass through brush more easily than high. Types used where rapids were to be run often had ends higher than the gunwales to prevent the canoe from shipping water over the bow. The high, distinctive ends of the canoes most used in the fur trade, on the other hand, were said to have resulted from the necessity of employing the canoe as a shelter. When the canoe was turned upside down on the ground, with one gunwale and the tops of the high ends supporting it, there was enough headroom under the canoe to permit its use as a shelter without the addition of any temporary structure. The desirability of this characteristic in the fur-trade canoe can be explained by the fact that the crew travelled as many hours as possible each day, and rested for only a very short period, so that rapid erection of shelter lengthened both the periods of travel and of rest. Yet these practical considerations do not always explain the end-forms found in bark canoes. Canoes with relatively high ends were used in open waters, and similar canoes were portaged extensively. Possibly the Indian's consciousness of tribal distinctions led him to retain some feature, such as height of the end-forms, as a means of tribal recognition, even though practical considerations required its suppression to some degree. The profile of the gunwales also varied a good deal among tribal types. Most bark canoes, because of the raised end-forms, showed a short, sharp upsweep of the sheer close to the bow and stern. Some showed a marked hump, or upward sweep, amidships which made the sheer profile follow somewhat the form of a cupid's bow. Many types had a straight, or nearly straight, sheer; others had an orthodox sheer, with the lowest part nearly amidships. The bottom profiles of bark canoes showed varying degrees of curvature. In some the bottom was straight for most of its length, with a slight rise toward the ends. In others the bottom showed a marked curvature over its full length, and in a few the bottom was practically straight between the points at which the stems were formed. Some northwestern types had a slightly hogged bottom, but in these the wooden framework was unusually flexible, so that the bottom became straight, or even a little rockered when the canoe was afloat and manned.