Little is known about temporal variability in nitrate concentration responses to changes in discharge on intraannual time scales in large rivers. To investigate this knowledge gap, we used a six-year data set of daily surface water nitrate concentration and discharge averaged from near-continuous monitoring at U.S. Geological Survey gaging stations on the Connecticut, Potomac, and Mississippi Rivers, three large rivers that contribute substantial nutrient pollution to important estuaries. Interannually, a comparison of nitrate concentration-discharge (c-Q) relationships between a traditional discrete grab sample data set and the near-continuous data set revealed differing c-Q slopes, which suggests that sample frequency can impact how we ultimately characterize hydrologic systems. Intraannually, we conducted correlation analyses over 30-day windows to isolate the strength and direction of monthly c-Q relationships. Monthly c-Q slopes in the Potomac were positive (enrichment/mobilization response) in summer and fall and negative (dilution response) and weakly chemostatic (nonsignificant near-zero c-Q slope) in winter and spring, respectively. The Connecticut displayed a dilution response year-round, except summer when it was weakly chemostatic. Mississippi c-Q slopes were weakly chemostatic in all seasons and showed inconsistent responses to discharge fluctuations. The c-Q dynamics in the Potomac and Connecticut were correlated (R > 0.3) to river temperature, flow percentile, and calendar day. Minimal correlation in the Mississippi suggests that the large basin area coupled with spatiotemporally variable anthropogenic forcings from substantial land use development created stochastic short-term c-Q relationships. Additional work using high-frequency sensors across large river networks can improve our understanding of spatial source input dynamics in these natural-human coupled systems. Key Points:• Monthly, seasonal, and annual nitrate concentration-discharge relationships vary in direction and strength across large rivers • Consistent nitrate-discharge relationship patterns driven in part by seasonality in biogeochemical processes and flow conditions • Nonchemostatic behavior in Mississippi River suggests complex heterogeneity in the timing and magnitude of source area connectivity Supporting Information:• Supporting Information S1

Height Above Nearest Drainage (HAND), a drainage normalizing terrain index, is a means able of producing flood inundation maps (FIMs) from the National Water Model (NWM) at large scales and high resolutions using reach‐averaged synthetic rating curves. We highlight here that HAND is limited to producing inundation only when sourced from its nearest flowpath, thus lacks the ability to source inundation from multiple fluvial sources. A version of HAND, known as Generalized Mainstems (GMS), is proposed that discretizes a target stream network into segments of unit Horton‐Strahler stream order known as level paths (LPs). The FIMs associated with each independent LP are then mosaiced together, effectively turning the stream network into discrete groups of homogeneous unit stream order by removing the influence of neighboring tributaries. Improvement in mapping skill is observed by significantly reducing false negatives at river junctions when the inundation extents are compared to FIMs from that of benchmarks. A more marginal reduction in the false alarm rate is also observed due to a shift introduced in the stage‐discharge relationship by increasing the size of the catchments. We observe that the improvement of this method applied at 4%–5% of the entire stream network to 100% of the network is about the same magnitude improvement as going from no drainage order reduction to 4%–5% of the network. This novel contribution is framed in a new open‐source implementation that utilizes the latest combination of hydro‐conditioning techniques to enforce drainage and counter limitations in the input data.

Flooding is one of the most significant natural disasters in the United States (US) affecting both the loss of life and property. In 2017 and 2019, river and flash flooding combined represented the leading cause of death and the second leading cause in 2018 among all natural disasters in the US (National Weather Service, 2018Service, 2020b). More than an average of 104 deaths per year are attributed to flood events from the 10 year period ending in 2019 (Service, 2020a). With respect to property damages, river and flash flooding have contributed to 60.7, 1.6, and 3.7 billion non-inflation adjusted US dollars in the annual periods of 2017-2019, respectively

Flood maps derived from Height Above Nearest Drainage (HAND) are subject to nearest drainage line limitations that affect inundation skill.• A means of resolving this limitation is provided by reducing HAND processing units to level paths with effective unit stream order.• Discretizing the stream network for HAND computation affects the stage-discharge relationship and leads to higher skill inundation.