Mind the gap: leveraging wind-gaps to identify competing river piracy events in southwestern Germany

Stream piracy has been central in explaining landscape evolution since W. M. Davis first introduced the concept. Reconstructions of drainage histories routinely invoke rerouting of an antecedent river to a lower adjacent stream. However, despite decades of analytical and computational progress, inferring discrete river reorganisation events remains challenging. In this contribution, we document how the transient drainage history of a region can be reconstructed using digital topography. Our premise is that previous topographic analyses neglect older stream piracy events. For example, in a typical retreating escarpment scenario, such as in southwestern Germany, erosion is concentrated in steep escarpment-draining rivers that occasionally capture plateau areas. These captures are readily detectable using topographic archives such as paired "area-gain/area-loss" profiles in χ-elevation space and mobile knickpoints at or upstream of capture points. However, such topographic archives decay as channels adjust to changes in drainage area, and thus many captures remain 'undetected' after escarpment retreat.

Here we use wind-gaps, a unique post-capture landform that is more prone to persist due to its position as a drainage divide, to identify otherwise 'undetectable' prior piracy events. We take advantage of TopoToolbox's DIVIDEobj algorithm to extract the drainage divide network of a landscape as a whole (i.e., every ridgeline separating neighbouring streams). From this, we calculate the ratio between the elevation of a segment in the divide network and the average elevation of neighbouring divides. We identify wind-gaps as (i) low-elevation divides confined on both sides by neighbouring higher divide segments, which (ii) are also characterised by low across-divide differences in relief. This approach provides insight into the drainage evolution history of South German Scarplands. The tectonic development of the Upper Rhine Graben led to an incipient northwest-oriented drainage that became progressively more erosive, especially since the Late Miocene. These northwest-draining rivers, such as the Neckar River, expanded their drainage areas via multiple discrete piracy events. This sequence of capture events led to the reversal of southern German rivers that originally drained to the southeast (towards the Danube). Our results identify tens of piracy events considerably downstream of the current divide separating the Neckar and Danube catchments that otherwise would not have been identified and put in temporal context. These results are in contrast to previous approaches that could only identify capture events in the vicinity of the current divide. In areas adjacent to wind-gaps and along 'reversed' and 'beheaded' streams, we explore the morphological relationships with the relative timing of the stream piracy events. Taken together, these results lead to a more comprehensive treatment of drainage history from topographic data.