Connections between agricultural runoff and excess nitrogen in the Upper Mississippi River Basin are well-documented, as is the potential role of constructed wetlands in mitigating this surplus nitrogen. However, limited knowledge exists about the "best" placement of these wetlands for downstream nitrogen reductions within a whole watershed context as well as how far downstream these benefits are realized. In this study, we simulate the cumulative impacts of diverse wetland restoration scenarios on downstream nitrate reductions in different subbasins of the Raccoon River Watershed, Iowa, USA, and spatially trace their relative effects downstream. Our simulated results underscore previous work demonstrating that the total area of wetlands and the wetland-catchment-to-wetland area ratio are both significant factors for determining the nitrate load reduction benefits of wetlands at subbasin scales. Simulated wetland conservation scenarios resulted in nitrate load decreases ranging from 7.5 to 43.2% of our baseline model loads. However, we found these wetland-mediated nitrate reduction benefits are quickly attenuated downstream: load reductions were <1% at the watershed outlet across all model scenarios, despite the magnitude of the subbasin-scale nitrate decreases. The relatively rapid attenuation of wetland effects is largely due to downstream nitrate load contributions from untreated subbasins. However, higher subbasin-scale nitrate reductions from wetland-based conservation practices resulted in longer downstream distances prior to attenuation. This study highlights the importance of considering the spatial location of constructed or restored wetlands relative to the area within the watershed where nitrogen reductions are most needed. The cumulative impact of this large agricultural expanse leads to increasingly high magnitude nitrate loads downstream throughout the basin, from the headwaters to the basin’s outlet in the Gulf of Mexico ( Tian et al., 2020 ). These surplus nitrate loads stimulate annual hypoxic “dead zones” in the Gulf, degrading water quality throughout the ecosystem and has shifted towards more eutrophication over decades of high nitrogen inputs ( Turner et al., 2008 ). In the northern portion of the Mississippi River Basin, particularly in the Prairie Pothole Region ( Fig. 1 ), agricultural lands situated on rich, glacial-till soils are poorly drained and thus require artificial subsurface drainage to maintain crop productivity ( Valayamkunnath et al., 2020 ). The wide-spread subsurface system of pipes or “tiles”, placed in parallel under fields, ultimately drain to surface waters (ditches and streams) and have been linked to greater nitrate loads both locally ( Blann et al., 2009 ; Gramlich et al., 2018 ) and within the greater Mississippi River Basin ( Arenas Amado et al., 2017 ; David et al., 2010 ). For example, although substantive wetland conservation-based practices in a given subbasin may affect nitrate levels at the pour-point of that subbasin, the signal (of the local effect) may be lost in the noise of the contributions from other downstream systems. This is analogous to the “house--neighborhood-community” approach to understanding the influence of conservation practices Modifying only one subbasin at a time allowed us to trace the downstream effects of wetland additions in each subbasin and assess how load reduction benefits would be mitigated by the contributions of other un-modified downstream subbasins. Additionally, the one subbasin at a time approach allows us to compare results of the same scenario among different subbasins within the Raccoon River to understand how the spatial location of added wetlands within a river basin affects the results both at the subbasin and watershed outlet scale. Without data to parameterize in-stream processes, this approach allows us to specifically focus on landscape-derived changes in nitrate loads from wetland construction. We evaluated the effectiveness of wetland construction as the difference in nitrate load at the outlet of the modified subbasin between the baseline model and each wetland-addition scenario. Since each subbasin has a different baseline load, these values are presented in percentage change from baseline to allow for comparisons across basins. These subbasin percentage changes were calculated at the outlet of the target subbasin and the outlet of each subsequent subbasin downstream all the way to the Raccoon River watershed outlet in Des Moines, Iowa ( Fig. 1 ). Additionally, for each scenario in each subbasin, we calculated the distance downstream, measured in the number of subbasins along the drainage network, that the reduction is more than 1% of the baseline load. Headwater basins, as defined here, refer to subbasins that do not receive any input from upstream basins. 3. Results 3.1. Second, the apparent downstream effectiveness of subbasin-scale wetland additions decreases with the number of upstream basins contributing nitrate loads. Non-floodplain wetlands receive inputs from the landscape via surface and subsurface (including tile) pathways; therefore, nitrate removal at the wetland, or local, scale is not affected by the stream network. However, when quantifying wetland nitrate-removal effectiveness at a subbasin scale (i.e., at the outlet of the subbasin), as we did here, upstream nitrate loads are added to the total nitrate loads at the subbasin outlet. While the three scenarios have varying impacts in the target subbasin (0), the differences start to converge around four subbasins downstream from the modified subbasin. Load reductions at the watershed outlet (18 subbasins downstream) are indistinguishable from zero. Fig. 4. Scatterplot showing the relationship between the number of upstream subbasins and the nutrient reduction benefits of wetland additions in each target subbasin, measured as % change from baseline loads for one scenario (Scenario 7). Each point is a subbasin.