No Snow_No Flow; The 2015 Snow Drought in the Oregon Cascades; Raw and Modeled Hydrologic and Geomorphic Data

Hydrologic extremes, such as drought, offer an exceptional opportunity to explore how runoff generation mechanisms and stream networks respond to changing precipitation regimes. The winter of 2014-2015 was the warmest on record in western Oregon, US, with record low snowpacks, and was followed by an anomalously warm, dry spring, resulting in historically low streamflows. But a year like 2015 is more than an outlier meteorological year. It provides a unique opportunity to test fundamental hypotheses for how montane hydrologic systems will respond to anticipated changes in amount and timing of recharge. In particular, the volcanic Cascade Mountains represent a “landscape laboratory” comprised of two distinct runoff regimes: the surface-flow dominated Western Cascade watersheds, with flashy streamflow regimes, rapid baseflow recession, and very low summer flows; and (b) the spring-fed High Cascade watersheds, with a slow-responding streamflow regime, and a long and sustained baseflow recession that maintains late summer streamflow through deep-groundwater contributions to high volume, coldwater springs. We hypothesize that stream network response to the extremely low snowpack and subsequent recharge varies sharply in these two regions. In surface flow dominated streams, the location of channel heads can migrate downstream, contracting the network longitudinally; the wetted channel width and depth contract laterally as summer recession proceeds and flows diminish. In contrast, in spring-fed streams, channel heads “jump” to the next downstream spring when upper basin spring flow diminishes to zero. Downstream of flowing springs, wetted channel width and depth contract laterally as flows recede. To test these hypotheses, we conducted a field campaign to measure changing discharge, hydraulic geometry, and channel head location in both types of watersheds throughout the summer and early fall. Multiple cross-section sites were established on 6 streams representing both flow regime types on either side of the Cascade crest. In addition we took Isotopic water samples to determine recharge elevations of receding streams. Taken together these measurements reveal the processes by which drainage networks contract as flows diminish – a fundamental property of montane stream systems both now and in the future.水文极端事件（hydrologic extremes），如干旱，为探究产流机制（runoff generation mechanisms）与河网（stream networks）对降水格局（precipitation regimes）变化的响应提供了绝佳契机。2014-2015年冬季为美国俄勒冈州西部有记录以来最温暖的冬季，积雪量（snowpacks）创下历史新低，随后而来的春季异常温暖干燥，导致径流量（streamflows）降至历史最低水平。 但2015年绝非仅有的异常气象年份，它为验证山地水文系统（montane hydrologic systems）如何响应预期的补给量与补给时间变化这一核心假说提供了独特契机。其中，喀斯喀特火山带（Volcanic Cascade Mountains）堪称“景观实验室”，其境内存在两种截然不同的产流模式：一是地表径流主导的西喀斯喀特流域，该区域径流暴涨暴落、基流退水（baseflow recession）迅速，夏季径流量极低；二是泉水补给型的高海拔喀斯喀特流域，其径流响应缓慢，基流退水过程漫长且稳定，依靠深层地下水补给高流量冷水泉水，从而维持夏季末期的径流量。 我们提出如下假说：在上述两类区域中，河网对极低积雪量及后续补给过程的响应存在显著差异。在地表径流主导的河流中，河源（channel heads）位置可向下游迁移，导致河网纵向收缩；随着夏季退水过程推进、径流量减少，湿润河道的宽度与深度也会横向收缩。与之相反，在泉水补给型河流中，当上游流域的泉水流量降至零时，河源会“跃迁至”下一处下游泉水处。在活跃泉水的下游区域，随着径流量退减，湿润河道的宽度与深度同样会横向收缩。 为验证上述假说，我们开展了野外实地观测，于整个夏季及初秋时段对两类流域的径流量变化、水力几何形态（hydraulic geometry）及河源位置进行测量。我们在喀斯喀特山脊两侧代表两类径流模式的6条河流上设置了多个断面测点（cross-section sites）。此外，我们采集了水体同位素样品（Isotopic water samples），以确定退水河段的补给高程（recharge elevations）。综合上述测量结果，可揭示径流量减小时河网收缩的物理过程——这是当前及未来山地河网系统的核心特性之一。