Robust design of detonation-based engines for propulsion or power generation applications requires understanding the behavior of detonation waves across different operating conditions. The height of fresh reactants that a detonation wave travels through is likely to impact its propagation behavior. With this motivation, this study evaluated the effect of changes in reactant fill heights on detonation behavior through hydrogen and air mixtures in channels. This was accomplished by attaching a straight detonation channel with optical access to a rotating detonation combustor (RDC). The RDC was operated independent of the channel, allowing for control of the reactant fill height in the channel. The radiation intensity emitted from water vapor (i.e., 2.61-2.81 μm) was measured using a high speed (30 kHz) mid-infrared (IR) camera. Wave speeds, wave angles, reactant fill heights, wave heights and peak radiation intensities were quantified. This study showed that minimum operability limits for detonations in channels in terms of fill height (h) normalized by mixture cell size (λ) were near h/λ = 1. Evidence of reactants not burning in detonation mode were identified at the lower equivalence ratios (i.e., 0.7-0.8). Higher fill heights correlate with larger amounts of reactants failing to detonate or deflagrate after a detonation wave passed, indicating lower efficiencies. Results of this study provide useful insights into the effects that fill height in an RDC can have on operability limits, combustion inefficiencies, and wave structure that exists inside the combustor.