Local scouring is a bed load transport phenomenon related to different scientific disciplines in Earth-science and hydraulic engineering. However, less attention is given on the enlargement processes of local scour holes at natural instream obstacles like boulders that are exposed to unsteady flow conditions in the course of a flood hydrograph. Experimental investigations in laboratory flumes offer the advantage that hydraulic boundary conditions can be systematically varied. This contribution yields novel experimental data on the impact of submergence ratio, hydrograph skewness, and flow intensity onto local scour holes at boulder-like obstacles and evaluates the effect of discharge chronologies, i.e. subsequent hydrographs in time onto the geometry of local scour holes. In total 48 unscaled flume experiments for subcritical, clear-water and live-bed conditions were performed. Experimental results reveal that (i) local scouring dominantly occurred at the rising limb when flow depth is comparable to the obstacle size so that the obstacle is unsubmerged. (ii) Up to 80% of the scour hole size at the end of experimental runs was reached in only 10% of the experimental runtime while the skewness of the hydrograph is important. The steeper the rising limb the quicker the local scour hole matured. (iii) Local scouring is history dependent and memory effects are present that describe the dependence on antecedent sizes of the scour hole. It is speculated that speculate that local scour holes found at boulders in the field may be used for the interpretation of the minimum duration of the beginning stage of a flood.