PRELIMINARY INVESTIGATION OF FLUVIAL LANDFORMS AND BED MATERIAL LOAD TRANSPORT IN 200 YEAR-OLD RUN-OF-THE -RIVER IMPOUNDMENTS ON GRAVEL-BED STREAMS OF THE CHRISTIANA RIVER BASIN, DELAWARE

The current paradigm of long term impoundments (+200 years) states that when dams remain in place, the impoundments become completely filled with sediment (considered to be fine-grained), and the sediment remains there until the dams are breached. While these ideas clearly apply to large dams, small run-of-river dams have received little evaluation through measurement or experimentation. We collected data from 9 impoundments formed behind 200 year-old run-of-the-river dams to describe the landforms created by dams and to infer their influence on bed material transport processes.

The dams in our study area have an average height of 1.5 m and average impoundment length of 1410 m. We define the maximum upstream influence of the dam by the first visible riffle above the impoundment (median grain size of 24-30 mm). Within the impoundments, the beds of the channels prograde from the heads of the impoundments downstream with the toe of the slope ~400 m upstream of the dam; these deposits are comprised mostly of sand (median grain size of <2-7 mm). Downstream of the progradational front, pools have formed with water depths ranging from 1.5-2.0 m. Just before the crest of the dam, sedimentation has created a ramp that drapes onto the crest and is coarser (median grain size 3-14 mm) than the preceding pool. Below the dam, scour pools and mid-channel bars are characteristic landforms. The scour pools begin immediately downstream from the dam and extend 10-23 m downstream. The mid-channel bars downstream range in length from 26-51 m, with median surface grain sizes from 3-45 mm.

These observations support two primary conclusions. First, the accommodation space behind the dam is not filled with sediment. Second, even the largest grain sizes transported by the river seem able to make it through the impoundment (this is demonstrated by large cobbles resting on the crest of the dam and the coarse mid-channel bars that exceed the volume of the scour pool downstream of the dam). We propose that the configuration we observe above, within, and below the impoundment likely represents a steady-state morphology because the impoundments have had ample time (~200 years) to adjust to the incoming bed material supplied from upstream.