A flume experiment was conducted to study channel adjustment to episodic sediment supply in mountain streams. The bulk sediment used for the bed and feed included grain sizes 0.5–64 mm with geometric mean D gfalse(bulkfalse) of 5.7 mm. Water discharge was held constant for 40 h, and 300 kg of sediment was supplied through a range of scenarios. Bed slope, sediment storage, sediment transport, and bed surface texture responded to sediment supply. During the first of seven runs, bed slope decreased from 0.022 m /m (flume slope) to 0.018 m /m due to sediment starvation. Bed slope increased beginning in the second run as the bed aggraded due to preferential storage of grains >8 mm. Transport rate and bed‐surface particle size were significantly affected by magnitude‐frequency of sediment feed. Under constant feed, transport rate increased gradually and D g false(surfacefalse) ranged between 12 and 15 mm. Instead, sediment pulses caused a pronounced increase in sediment transport rate and surface fining, trends that were inverted as sediment evacuated. At the run scale, sediment transport and storage behaved as with constant feed if pulse relaxation time exceeded time between pulses. The increase in transport rate and surface fining were proportional to pulse size. After the 300 kg pulse, transport rate reached 100  g m−1 s−1 and D g false(surfacefalse) was <10 mm. After 75 kg pulses, transport rate reached ∼10  g m−1 s−1 and D g false(surfacefalse) was >12 mm. Textural differences on the initial bed surface influenced the patterns of sediment transport. Channel adjustment was controlled by magnitude‐frequency of sediment feed and not by total feed.

The effect of sediment supply on sediment mobility is analyzed for a poorly sorted (0.5–64 mm) experimental bed. Water discharge was held constant over a sequence of seven runs, and 300 kg of sediment was supplied during each run in different magnitudes and frequencies. In runs with constant feed bedload transport rate increased gradually. In contrast, runs that received large sediment pulses showed pronounced increases in bedload rate as the bed surface got finer, followed by monotonic declines as the bed surface coarsened. We studied the temporal scales of bedload fluctuations by means of sample autocorrelation coefficients and the rates of decrease in bedload fluctuation with sampling time scale. The significant trends caused in bedload rate by large occasional sediment pulses increased long‐term autocorrelation in bedload rate time series relative to runs with constant feed. Bed evolution and local changes in sediment storage caused multiple scales of variability in bedload rate, which increased autocorrelation and caused long‐term persistence in bedload series over periods with a nearly constant mean. The scaling statistics of bedload transport fluctuation depended on grain size, and those for total bedload were similar to those for fine gravel (2–8 mm), which was fully mobile and dominated bedload transport. Grain size dependence of bedload fluctuation was not affected by changes in sediment feed because water discharge and sediment texture were held constant.

No abstract