Sediment often enters rivers in discrete pulses associated with landslides and debris flows. This is particularly so in the case of mountain streams. The topographic disturbance created on the bed of a stream by a single pulse must be gradually eliminated if the river is to maintain its morphological integrity. Two mechanisms for elimination have been identified: translation and dispersion. According to the first of these, the topographic high translates downstream. According to the second of these, it gradually diffuses away. In any given river both mechanisms may operate. This paper is devoted to a description of three controlled experiments on sediment pulses designed to model conditions in mountain streams. Each of the experiments began from the same mobile‐bed equilibrium with a set rate and grain size distribution of sediment feed. In one experiment the median size of the pulse material was nearly identical to that of the feed sediment. In the other two the pulse material differed in grain size distribution from the feed sediment, being coarser in one case and finer in the other case. In all cases the mode of pulse deformation was found to be predominantly dispersive, a result that constitutes the main conclusion of this paper. The pulses resulted in a notable but transient elevation of sediment transport rate immediately downstream. When the pulse was coarser than the ambient sediment, the bed downstream remained armored, and a migrating delta formed in the backwater upstream. When the pulse was finer than the ambient sediment, translation was observed in addition to dispersion. The presence of the finer material notably elevated the transport of ambient coarse material on the bed downstream. In part 2 [Cui et al., 2003], the experimental results are used to test a numerical model.

We surveyed adjacent reaches with differing riparian vegetation to explain why channels with forested banks are wider than channels with nonforested banks. Cross sections and geomorphic mapping demonstrate that erosion occurs at cutbanks in curving reaches, while deposition is localized on active fl oodplains on the insides of bends.Our data indicate that rates of deposition and lateral migration are both higher in nonforested reaches than in forested reaches. Two dimensionless parameters, α and E, explain our observations. α represents the infl uence of grassy vegetation on rates of active fl oodplain deposition; it is 5 times higher in nonforested reaches than in forested reaches. E is proportional to rates of cutbank migration; it is 3 times higher in nonforested reaches than in forested reaches. Differences in width between forested and nonforested reaches are proportional to E/α. In forested reaches, channels are wide with banks that are diffi cult to erode. Dense tree roots create a low value of E, and the channel migrates slowly. E/α is high, however, because α is very low: shade from trees inhibits the growth of grass on active fl oodplains. In nonforested reaches, channels are narrow with banks that are easy to erode. E is high, and the channel migrates rapidly. E/α is low, however, due to a very large value of α: grass grows readily on nonforested convex bank fl oodplains. Thus, differences in width between forested and nonforested reaches are related to a balance between rates of cutbank erosion and rates of deposition on active fl oodplains, implying that equilibrium widths develop to equalize rates of cutbank erosion and vegetationmediated rates of deposition on active fl oodplains. These results suggest that accurate models of width adjustment should consider the combined effects of bank erodibility and fl oodplain depositional processes, rather than focusing on these processes in isolation from one another.

Despite widespread interest, few sediment budgets are available to document patterns of erosion and sedimentation in developing watersheds. We assess the sediment budget for the Good Hope Tributary, a small watershed (4.05 km2) in Montgomery County, Maryland, from 1951‐1996. Lacking monitoring data spanning the period of interest, we rely on a variety of indirect and stratigraphic methods. Using regression equations relating sediment yield to construction, we estimated an upland sediment production of 5,700 m3 between 1951 and 1996. Regression equations indicate that channel cross‐sectional area is correlated with the extent of development; these relationships, when combined with historical land use data, suggest that upland sediment yield was augmented by 6,400 m3 produced by enlargement of first‐order and second‐order stream channels. We used dendrochronology to estimate that 4,000 m3 of sediment was stored on the floodplain from 1951‐1996. The sediment yield from the watershed, obtained by summing upstream contributions, totals 8,100 m3 of sediment, or 135 tons/km2/year. These results indicate that upland erosion, channel enlargement, and floodplain storage are all significant components of the sediment budget of our study area, and all three are approximately equal in magnitude. Erosion of “legacy” floodplain sediments originally deposited during poor agricultural practices of the 19th and early 20th Centuries has likely contributed between 0 and 20% of the total sediment yield, indicating that these remobilized deposits are not a dominant component of the sediment yield of our study area.

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We predict changes in bed elevation and grain size composition caused by urbanization from 1952 to 1996 in the channel network of the Good Hope Tributary watershed. We developed methods for predicting the influence of urbanization on (1) the 1.5‐year peak discharge, (2) the annual sediment supply to the network, and (3) sediment production caused by channel enlargement. The model was calibrated to reproduce bed material yield estimated from a sediment budget. Development caused channel width to increase by a factor of 1.7, and discharge, upland sediment supply, and bed material yield approximately doubled. The longitudinal profile became smoother, and the bed coarsened. After 1996, model boundary conditions were held constant and the simulation continued through 2042. The bed continued to coarsen, and the yield of bed material gradually declined. Sediment production did not approach a steady state value, suggesting that geomorphic recovery from urbanization may require more than 50 years.