Steep streams occupy a large fraction of mountainous drainage basins and partially control the sediment supplied to downstream rivers. In these channels, sediment transport equations typically over-predict bedload flux by several orders of magnitude because they do not account for sediment-supply limited conditions. Thus, accurate predictions of bedload flux require an estimate of the sediment available for transport in a given event. We demonstrate through field measurements that boulder step protrusion is a proxy for sediment availability. Protrusion is also a function of the time elapsed since an extreme event and this simple relationship can be used to estimate the relative sediment availability at any given time. In addition, bedload transport predictions in a steep channel were only accurate if they included this variable protrusion. Predictions of sedimentation hazards, water quality, river restoration success, long-term channel network evolution, and channel stability may therefore require estimates of sediment availability for transport.