Headwater streams change in wet length in response to storm events and seasonal moisture conditions. These low-order channels with temporary flow are pervasive across arid and humid environments yet receive little attention in comparison to perennial waterways. This dissertation examines headwater stream length dynamics at multiple spatial and temporal scales across the Appalachians. I mapped wet stream length in four Appalachian physiographic provinces--the Appalachian Plateau, Blue Ridge, New England, and Valley and Ridge--to characterize seasonal expansion and contraction of the wet network at a broad, regional scale. Conversely, most existing field studies of stream length in headwaters are limited to a single study area or geographic setting. Field mappings showed that wet stream length varies widely within the Appalachians; network dynamics correlated with regional geology as well as local site lithology, geologic structure, and the depth, size, and spatial distribution of surficial sediment deposits. I used the field data to create logistic regression models of the wet network in each physiographic province at high and low runoffs. Topographic metrics derived from elevation data were able to explain the discontinuous pattern of headwater streams at different flow conditions with high classification accuracy. Finally, I used flow intermittency sensors in a single Valley and Ridge catchment to record channel wetting and drying at a high temporal resolution. The sensors indicated stream length hysteresis during storms with low antecedent moisture, with a higher wet network proportion on the rising limb than on the falling limb of events. As a result, maximum network extension can precede peak runoff by minutes to hours. Accurate maps of headwater streams and an understanding of wet network dynamics through time are invaluable for applications surrounding watershed management and environmental policy. These findings will contribute to the burgeoning research on temporary streams and are additionally relevant for studies of runoff generation, biogeochemical cycling, and mass fluxes of material from headwaters. / Ph. D. / During a rain storm, we may think of streams increasing in depth, width, and velocity. However, we may not necessarily envision streams also getting longer. Headwaters, which form the upstream extremities of river systems, consist of many temporary streams that expand and contract in length due to storms and changes in seasonal moisture conditions. Headwaters are spatially expansive, comprising a majority of total river length, and serve as a primary control on downstream water quality. Therefore, understanding stream length dynamics can inform policy and land use decisions to effectively conserve and manage headwater regions and protect water sources for human use and consumption. This dissertation examines changes in stream length across four study areas of the Appalachian Mountains. I mapped the wet, or active, stream network multiple times at different flow conditions in each study area. Stream length dynamics varied considerably across the Appalachians and demonstrated the same range of network expansion and contraction as other studies observed in diverse settings around the world. Wet stream length greatly depended on regional and local geology. I then sought to predict the location of wet streams at high and low flows using metrics such as slope and drainage area that I calculated from digital elevation information. Comparisons with the field maps I made showed that simple terrain metrics explained the location, length, and disconnected nature of wet networks in each province with high accuracy. I also observed stream length dynamics during storm events in one watershed using sensors that recorded the presence or absence of water. These observations demonstrated that stream length was often higher for a given flow at the beginning of a storm on the rising limb than on the falling limb when flow was decreasing, particularly if conditions were dry before the storm. The findings of this dissertation contribute to existing knowledge of temporary streams and are relevant for future studies investigating the hydrology, biology, and ecology of headwaters.