4.4.2 Case 4 Tributary Response
Complexities in the trunk response again dramatically affect the tributary responses, and generally mirror complexities discussed in Case 3. Tributaries again respond to variable and commonly conflicting signals. In this case, tributaries downstream fromxsc initially respond to a decrease in rainfall (lower ­ erosional efficiency) by steepening. However, decreases in erosional efficiency along the trunk profile drives steepening and surface uplift, forcing tributaries to respond to a conflicting signal of base-level rise (Movie S4). Additionally, tributaries in downstream locations are relatively much drier and adjust relatively slowly compared to upstream locations. Interestingly, the trunk profile adjusts and communicates transient base-level signals upstream to wetter tributaries relatively quickly compared to the adjustment timescale of these dry tributaries, which follows as the trunk river has much higher erosional efficiency from rainfall concentrated in its headwaters. The initially counterintuitive result of this is that central portions of the catchment are the first to achieve steady state (i.e., both the trunk profile and tributaries achieve the new steady state), followed by the headwaters, and lastly tributaries near the outlet where the transient signal originated, which contrasts with the common expectation of adjustment proceeding in an upstream fashion.