The huge boulder has a crescent shape to it with scour marks where the water ran around it during the flood.  Stop 3 in Baker et al. 

"This immense boulder (Fig. 21) measures ~18 × 11 × 8 m. It has a prominent crescent-shaped scour mark (now mostly filled with talus) cut into the sediment on the northwest (upstream) side of the boulder, and an elongate elliptical scour hole on the southeast (downstream) side. This scour pattern can be explained by a dual vortex system generated by the blunt obstacle posed by the boulder to the megaflood flow field during the waning stages of a flood (Baker, 1973a, 1978c; Baker and Komar, 1987). The size of this boulder attests to the transport capability of the megaflood waters (Fig. 10) as they poured through the Soap Lake constriction point. The boulder consists of basalt entablature, the interlocking jointing of which likely led to its remaining intact during transport from outcrops that lie ~8 km to the north. The many other large boulders in this area are smaller, which may be the result of their inability to remain intact during transport. An alternative hypothesis is that this boulder was ice-rafted into the present location. However, there are many problems with that hypothesis that will be further discussed on the trip."