Drivers of Dynamic Disconnectivity in Rivers

Dynamic Connectivity in the Landscape
Chair: Adam Ward (Indiana University)
Connectivity between different locations on the landscape is defined by the movement of water, solutes, energy, and organisms. The magnitude and persistence of connections is critical to prediction of ecological functions, many of which are mediated by hydrological stores and fluxes. In this session we consider connectivity as a spatially and temporally variable process in catchments and river systems.

"Drivers of Dynamic Disconnectivity in Rivers
Speaker: Ellen Wohl (Colorado State University)

As the science of hydrologic connectivity has developed, there is sometimes a tendency to under-emphasize the importance of disconnectivity. Connectivity and disconnectivity can both be characterized in terms of magnitude, frequency, duration, timing, directionality, and dimension. The latter characteristic is important because features that create longitudinal disconnectivity in rivers, for example, can enhance lateral and vertical connectivity. Here, I discuss naturally occurring processes that limit longitudinal connectivity in river corridors and the effect of these longitudinal disconnections on river form and function. Examples of such processes include lateral channel movement and the associated secondary channels, avulsions and cutoffs; lateral sediment inputs from tributaries or adjacent hillslopes that create alluvial fans in the river corridor; logjams; and beaver dams. I focus on logjams and beaver dams using case studies from mountain streams in the Southern Rockies. River networks in this high-relief terrain are predominantly steep, narrow canyons with high longitudinal connectivity and limited lateral connectivity between channels and floodplains or vertical connectivity between channels-floodplains and the hyporheic zone. Wide, low-gradient valley segments scattered throughout the network provide retention zones in mountainous river networks and typically exhibit greater lateral and vertical connectivity than the intervening steep, narrow segments. The details of connectivity, however, and the magnitude of retention and the partitioning of retained water, solutes, sediment, and organic matter among alluvial storage and atmospheric emissions depend in large part on the presence of features that limit longitudinal connectivity, as such as logjams or beaver dams. Through research conducted during the past decade, we have found that channel-spanning logjams and beaver dams substantially decrease downstream transport of water, solutes, sediment, and organic matter during both base flows and snowmelt peak flows. At the same time, logjams and beaver dams substantially increase channel-floodplain and channel-hyporheic exchange and thereby increase retention and long-term (102-103 year) alluvial storage of sediment and organic carbon. One implication of these findings is that river management can be designed to limit longitudinal connectivity in ways that foster ecosystem services such as flood attenuation, reduction of downstream sediment, nitrate, and carbon fluxes, and increased habitat abundance and diversity.

景观动态连通性

主席：亚当·沃德（印第安纳大学）

景观中不同地点之间的连通性由水、溶质、能量和生物体的运动所定义。连接的强度和持久性对于预测生态功能至关重要，其中许多功能是由水文储存和通量所介导的。在本会议中，我们将连通性视为一个在流域和河流系统中具有空间和时间变化性的过程。

河流动态断连的驱动因素
演讲者：艾伦·沃尔（科罗拉多州立大学）

随着水文连通性科学的发展，有时会出现一种趋势，即低估断连的重要性。连通性和断连都可以用幅度、频率、持续时间、时间、方向性和维度等术语来表征。后者的特征尤为重要，因为那些在河流中造成纵向断连的特征，例如，可以增强横向和垂直连通性。在此，我将讨论自然发生的那些限制河流走廊纵向连通性的过程，以及这些纵向断连对河流形态和功能的影响。此类过程包括横向河道移动及其相关的次级河道、侵蚀和截流；来自支流或相邻山地的横向沉积物输入，这些沉积物在河流走廊中形成冲积扇；阻塞；以及海狸坝。我专注于阻塞和海狸坝，使用来自南落基山脉山区溪流的案例研究。在这个高海拔地形中，河流网络主要由陡峭、狭窄的峡谷组成，纵向连通性高，而河道与河漫滩或河道-河漫滩与潜流区之间的横向连通性和垂直连通性有限。散布在整个网络中的宽阔、低坡度的山谷段为山区河流网络提供了滞留区，并且通常比其间的陡峭、狭窄段表现出更大的横向和垂直连通性。然而，连通性的具体细节以及滞留量和滞留的水、溶质、沉积物和有机物在冲积储存和大气排放之间的分配，在很大程度上取决于限制纵向连通性的特征的存在，如阻塞或海狸坝。在过去十年间的研究中，我们发现，跨越河道的阻塞和海狸坝在基流和融雪峰值流量期间都会显著减少水、溶质、沉积物和有机物的下游运输。同时，阻塞和海狸坝显著增加了河道-河漫滩和河道-潜流区的交换，从而增加了滞留和长期（102-103年）的冲积储存和有机碳的滞留。这些发现的一个含义是，河流管理可以设计成限制纵向连通性，从而促进生态系统服务，如洪水缓解、下游沉积物、硝酸盐和碳通量的减少，以及栖息地丰富度和多样性的增加。