Advances in near-real-time forecasts of lava flow advance rates, final travel distance, and areal coverage are transforming hazard response, particularly in locations such as Hawaiʻi that experience frequent lava flow activity. The most severe threats are posed by rapidly advancing channelized ʻaʻā flows, such as those that characterized the November–December 2022 eruption of Mauna Loa volcano. To constrain rheological inputs to flow forecasting models during the eruption, samples were collected from the two most persistent lava flows and rapidly assessed for downflow changes in crystallinity and vesicularity. As observed in other channelized lava flows, the lava lost bubbles and cooled during transport, with consequent increases in groundmass crystallinity and bulk viscosity. Lava rheology, however, is controlled by more than bulk bubble and crystal contents; also important are the melt viscosity and the size and shape distributions of bubbles and crystals. Here we further interrogate the sample suite to document a down flow increase of almost two orders of magnitude in melt viscosity alone (a function of cooling combined with compositional change) and assess the extreme anisotropy of the plagioclase crystals as a function of size and flow type (pāhoehoe or ʻaʻā). When combined with previous studies, these data constrain observed relations between surface morphology, flow temperature and crystal content and indicate that remote assessments of morphological transitions and core lava temperature could be used for channel-wide monitoring of rheological evolution for assimilation into flow models.