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We present a novel quantitative test of a 50‐year‐old hypothesis which asserts that river delta morphology is determined by the balance between river and marine influence. We define three metrics to capture the first‐order morphology of deltas (shoreline roughness, number of distributary channel mouths, and presence/absence of spits), and use a recently developed sediment flux framework to quantify the river‐marine influence. Through analysis of simulated and field deltas we quantitatively demonstrate the relationship between sediment flux balance and delta morphology and show that the flux balance accounts for at least 35% of the variance in the number of distributary channel mouths and 42% of the variance in the shoreline roughness for real‐world and simulated deltas. We identify a tipping point in the flux balance where wave influence halts distributary channel formation and show how this explains morphological transitions in real world deltas.

 

Human activities have increased nitrate export from rivers, degrading coastal water quality. At deltaic river mouths, the flow of water through wetlands increases nitrate removal, and the spatial organization of removal rates influences coastal water quality. To understand the spatial distribution of nitrate removal in a river‐dominated delta, we deployed 23 benthic chambers across ecogeomorphic zones with varying elevation, vegetation, and sediment properties in Wax Lake Delta (Louisiana, USA) in June 2018. Regression analyses indicate that normalized difference vegetation index is a useful predictor of summertime nitrate removal. Mass transfer velocity were approximately three times greater on a vegetated submerged levee (13 mm hr⁻¹), where normalized difference vegetation index was greatest, compared to other locations (4.6 mm hr⁻¹). Two methods were developed to upscale nitrate removal across the delta. The flooded‐delta method integrates spatially explicit potential removal rates across submerged portions of the delta and suggests that intermediate elevations on the delta—including submerged levees—are responsible for 70% of potential nitrate removal despite covering only 33% of the flooded area. The channel network method treats the delta as a network of river channels and suggests that although secondary channels are more efficient than primary channels at removing received nitrate, primary channels collectively contribute more to overall removal because they convey more of the total nitrate load. The two upscaling methods predict similar rates of nitrate removal, equivalent to less than 4% of nitrate entering the delta. To protect coastal waters against high nitrate loads, management policies should aim to reduce upstream nutrient loads.

 

Abstract Climate change is intensifying tropical cyclones, accelerating sea-level rise, and increasing coastal flooding. River deltas are especially vulnerable to flooding because of their low elevations and densely populated cities. Yet, we do not know how many people live on deltas and their exposure to flooding. Using a new global dataset, we show that 339 million people lived on river deltas in 2017 and 89% of those people live in the same latitudinal zone as most tropical cyclone activity. We calculate that 41% (31 million) of the global population exposed to tropical cyclone flooding live on deltas, with 92% (28 million) in developing or least developed economies. Furthermore, 80% (25 million) live on sediment-starved deltas, which cannot naturally mitigate flooding through sediment deposition. Given that coastal flooding will only worsen, we must reframe this problem as one that will disproportionately impact people on river deltas, particularly in developing and least-developed economies. 

Abstract. River deltas are sites of sediment accumulation along thecoastline that form critical biological habitats, host megacities, andcontain significant quantities of hydrocarbons. Despite their importance, wedo not know which factors most significantly promote sediment accumulationand dominate delta formation. To investigate this issue, we present a globaldataset of 5399 coastal rivers and data on eight environmental variables.Of these rivers, 40 % (n=2174) have geomorphic deltas defined eitherby a protrusion from the regional shoreline, a distributary channel network,or both. Globally, coastlines average one delta forevery ∼300 km of shoreline, but there are hotspots of delta formation, for examplein Southeast Asia where there is one delta per 100 km of shoreline. Ouranalysis shows that the likelihood of a river to form a delta increases withincreasing water discharge, sediment discharge, and drainage basin area. Onthe other hand, delta likelihood decreases with increasing wave height andtidal range. Delta likelihood has a non-monotonic relationship withreceiving-basin slope: it decreases with steeper slopes, but for slopes >0.006 delta likelihood increases. This reflects differentcontrols on delta formation on active versus passive margins. Sedimentconcentration and recent sea level change do not affect delta likelihood. Alogistic regression shows that water discharge, sediment discharge, waveheight, and tidal range are most important for delta formation. The logisticregression correctly predicts delta formation 74 % of the time. Our globalanalysis illustrates that delta formation and morphology represent a balancebetween constructive and destructive forces, and this framework may helppredict tipping points at which deltas rapidly shift morphologies.