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.