Joshua B. Logan

Quantitative morphology of a fringing reef tract from high-resolution laser bathymetry: Southern Molokai, Hawaii

High-resolution Scanning Hydrographic Operational Airborne Lidar Survey (SHOALS) laser-determined bathymetric data were used to define the morphology of spur-and-groove structures on the fringing reef off the south coast of Molokai, Hawaii. These data provide a basis for mapping and analyzing morphology of the reef with a level of precision and spatial coverage never before attained. An extensive fringing coral reef stretches along the central two-thirds of Molokai9s south shore (∼40 km); along the east and west ends there is only a thin veneer of living coral with no developed reef complex. In total, ∼4800 measurements of spur-and-groove height and the distance between adjacent spur crests (wavelength) were obtained along four isobaths. Between the 5m and 15m isobaths, the mean spur height increased from 0.7 m to 1.6 m, whereas the mean wavelength increased from 71 m to 104 m. Reef flat width was found to exponentially decrease with increasing wave energy. Overall, mean spur-and-groove height and wavelength were shown to be inversely proportional to wave energy. In high-energy environments, spur-and-groove morphology remains relatively constant across all water depths. In low-energy environments, however, spur-and-groove structures display much greater variation; they are relatively small and narrow in shallow depths and develop into much larger and broader features in deeper water. Therefore, it appears that waves exert a primary control on both the small- and large-scale morphology of the reef off south Molokai.

Supply and dispersal of flood sediment from a steep, tropical watershed: Hanalei Bay, Kaua’i, Hawai’i, USA

In contrast to many small, mountainous watersheds in temperate coastal regions, where fl uvial discharge and wave energy commonly coincide, deposition and reworking of tropical fl ood sediment can be seasonally decoupled, and this has important implications for coral-reef ecosystems. An understanding of the interaction between tropical fl ood sedimentation and wave climate is essential to identifying and mitigating effects of watershed changes on coral reefs as urbanization and climate change proceed. Sedimentary facies and isotopic properties of sediment in Hanalei Bay, on the island of Kaua’i, Hawai’i, USA, were used to assess deposition and reworking of fldeposits from the Hanalei River in a case study demonstrating the potential ecosystem effects of runoff from a steep, tropical watershed. In Hanalei Bay, the youngest and thickest terrigenous sediment was consistently present near the river mouth and in a bathymetric depression that acted as at least a temporary sediment sink. During this 2 yr study, the largest fl ood events occurred in late winter and spring 2006; substantial terrestrial sediment delivered by those fl oods still remained in the bay as of June 2006 because oceanic conditions were not suffi ciently energetic to transport all of the sediment offshore. Additional sediment was deposited in the bay by a summer 2006 fl ood that coincided with seasonal low wave energy. In most years, fl ood sediment accumulating in the bay and on its fringing reefs would be remobilized and advected out of the bay during winter, when the wave climate is energetic. Turbidity and sedimentation on corals resulting from late spring and summer fl oods during low wave energy could have a greater impact on coral-reef ecosystems than fl oods in other seasons, an effect that could be exacerbated if the incidence and sediment load of tropical summer fl oods increase due to urbanization and climate change.

River response to large-dam removal in a Mediterranean hydroclimatic setting: Carmel River, California, USA: River response to large-dam removal

Dam removal provides a valuable opportunity to measure the fluvial response to changes in both sediment supply and the processes that shape channel morphology. We present the first study of river response to the removal of a large (32-m-high) dam in a Mediterranean hydroclimatic setting, on the Carmel River, coastal California, USA. This before-after/control-impact study measured changes in channel topography, grain size, and salmonid spawning habitat throughout dam removal and subsequent major floods. During dam removal, the river course was re-routed in order to leave most of the impounded sediment sequestered in the former reservoir and thus prevent major channel and floodplain aggradation downstream. However, a substantial sediment pulse occurred in response to base-level fall, knickpoint migration, and channel avulsion through sediment in the former reservoir above the newly re-routed channel. The sediment pulse advanced ~3.5 km in the first wet season after dam removal, resulting in decreased riverbed grain size downstream of the dam site. In the second wet season after dam removal, high flows (including a 30-year flood and two 10-year floods) transported sediment > 30 km downstream, filling pools and reducing cross-channel relief. Deposition of gravel in the second wet season after dam removal enhanced salmonid spawning habitat downstream of the dam site. We infer that in dam removals where most reservoir sediment remains impounded and where high flows follow soon after dam removal, flow sequencing becomes a more important driver of geomorphic and fish-habitat change than the dam removal alone.