Headwaters, defined here as first- and second-order streams, make up 70%–80% of the total channel length of river networks. These small streams exert a critical influence on downstream portions of the river network by: retaining or transmitting sediment and nutrients; providing habitat and refuge for diverse aquatic and riparian organisms; creating migration corridors; and governing connectivity at the watershed-scale. The upstream-most extent of the channel network and the longitudinal continuity and lateral extent of headwaters can be difficult to delineate, however, and people are less likely to recognize the importance of headwaters relative to other portions of a river network. Consequently, headwaters commonly lack the legal protections accorded to other portions of a river network and are more likely to be significantly altered or completely obliterated by land use.
We are unable to predict the location of first-order channels in particular, which limits ability to define their spatial distribution and cumulative length or area. This in turn limits understanding of their physical or ecological function, as well as the extent of direct or indirect human alteration of these smallest channels in a river network
Resistance & resilience
We cannot adequately characterize or predict either physical or ecological resistance or resilience of many headwater streams. We need additional case studies of headwaters response to natural and human-induced disturbances in order to develop a sufficient body of literature to support generalizations and predictions. We also need metrics to characterize resistance and resilience of headwater streams, which are likely to have greater magnitude and frequency of disturbances than higher-order channels
Human alterations
We need more site-specific case studies and watershed- to regional-scale assessments of how land use and climate change affect the distribution, form, and function of headwaters
Biota
Complete species inventories do not exist for the great majority of headwater streams, which limits our ability to understand and predict the contribution of headwaters to biodiversity and the ecological resilience of these streams to natural and human disturbances
Hydrology
There is a need for both techniques to measure the spatial and temporal extent of surface flow in streams that are ephemeral or intermittent, and datasets of such measurements. We also need more quantitative measurements of hydrologic variability and metrics that characterize hydrologic variability.
Hydraulics and sediment regime
Headwaters are predominantly sediment transfer zones, but we need more case studies of the magnitude and episodicity of sediment inputs and downstream transfer, including the hydraulic thresholds that facilitate such transfer.
Connectivity
As with hydrologic variability, we need more measurement techniques, metrics, and datasets of diverse forms of connectivity in headwaters.
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