GEOMORPHIC COMPARISON BETWEEN THE SUN RIVER, MONTANA AND SOPOCHNAYA RIVER, KAMCHATKA, RUSSIA: IMPLICATIONS FOR FLOODPLAIN FUNCTION AND LAND USE

The comparison of Holocene geomorphic features in the Sun River watershed with those of the Sopochnaya River in remote Kamchatka, Russia underscores major differences in floodplain function related to land use and flow regulation. Three major Holocene fluvial terraces are evident in the alluvium of the Sun River, Montana probably related to Holocene climatic changes. The Pleistocene terrace sits 30’ above bankfull stage and formed on deformed Cretaceous shale. The highest Holocene terrace stands 15’ above bankfull stage. The middle terrace is found 6’-8’ above bankfull and is rarely inundated (>100 year flood events). This feature has well-developed loamy soil and has been deforested and farmed along the Sun River. The lowest terrace is 3’ above bankfull and is inundated more frequently (10 year flood events). The lower terrace supports 150 year old decadent cottonwood trees. The Sopochnaya River in Kamchatka, in contrast, has a similar geomorphic setting but the watershed has no roads, logging or grazing. Pleistocene terraces are developed on tilted marine shales and Holocene terrace levels are similar to the Sun River. However, in contrast to the Sun River, there is no lower Holocene terrace. Instead, the low landforms associated with the Sopochnaya are active floodplains. These features are widespread and are frequently inundated (1-2 times per year). Floodplain vegetation is young (<20 years old) and soils are loamy and organic rich. The implications for the comparison of the Holocene geomorphic development between the Sun River, and the Sopochnaya River, are: 1) Flow diversion and alteration caused the Sun River to down-cut throughout its length, within the past 150 years (decadent cottonwood ages). 2) During this time the Sun River has lost access to a wide floodplain, the floodplain vegetation has become decadent and the floodplain soils have dessicated. 3) The large floods that do occur on the Sun River, strip dry soils and erode the floodplain as opposed to the more frequent inundation of the floodplain on the Sopochnaya. 4) Although remarkably similar in many respects, the contrast in land use and flow regulation between the Sun River and the Sopochnaya results in major differences in floodplain function.