Responses of headwater stream networks to hydrological variability and extreme drought, including the role of geology, have not been well quantified. The effects of drainage area and subsurface conveyance capacity ( SCC ) on stream wetted cross-sectional area ( A wxs ) and probability of stream disconnection were quantified in a mountainous headwater watershed of coastal Northern California under seasonal variations and extreme drought in 2021. Field measurements of wetted characteristics, including A wxs and percentage of the reach that was dry, were made during the dry season. A wxs correlated negatively with SCC and positively with drainage area in the log-transformed regression. A wxs decreased and probability of disconnection ( P ( d ) ) increased with increasing distance from the headwaters due to high SCC in the lower reaches. Drainage area and SCC were both statistically significant predictors of A wxs , particularly in multivariate modes ( p << .01, R² = 0.4). Drainage area was more influential during wet conditions, while SCC was critical during dry periods. In logistic models, SCC was a stronger predictor of P ( d ) than drainage area ( p < 0.001). It is herein concluded that geology within this watershed is the dominant control on surface water expression during extreme drought conditions, resulting in unexpectedly decreasing wetted channel dimensions with increasing distance downstream. This dataset presents a unique opportunity to examine stream responses during critical drought conditions in a headwaters watershed that supports spawning and rearing habitat for endangered anadromous fish. Findings can help predict stream responses in similar watersheds to forecasted hydrological conditions and mitigate potential impacts to aquatic habitat.