Long-term Limnological and Aquatic Resource Monitoring for Lakes Mead and Mohave
Photo by Michael Rosen, USGS
After the completion of Hoover Dam in 1935, sediment began accumulating in the new reservoir (Lake Mead) as the flow of the Colorado River was captured. Three studies have examined the amount of sediment accumulation and the rate of sedimentation in Lake Mead: a 1948-1949 USGS and Reclamation study (Smith et al. 1960), a 1963-1964 Reclamation study after the closure of Glen Canyon Dam (Lara and Sanders 1970), and a 2001 USGS and Reclamation study. Results of the 2001 study show that post-impoundment sediment is generally distributed along the floors of the deepest parts of the lake, mainly following the paths of the former Colorado River and the tributary valleys that fed into it, rather than being deposited as a drape across the entire lake floor (Twichell et al. 2005). These sediments are thickest in the deltas that formed at the mouths of the Colorado River and its tributaries, including the Virgin and Muddy Rivers. Maximum sediment thickness exceeds 262 ft where the Colorado River enters Lake Mead, thinning to 50-115 ft in thickness along the remainder of the drowned Colorado River channel to Hoover Dam. Tributary valleys have a thinner sediment cover indicating the Colorado River has been the primary sediment source. Sediment cores indicated stratification of fine silt interrupted by graded beds containing as much as 30 percent sand deposited from turbidity currents, which flowed the full length of the lake. With the completion of Glen Canyon Dam in 1964, sediment volume entering Lake Mead from the Colorado River decreased to approximately one tenth of the pre-dam volume (Lara and Sanders 1970). With the lake levels dropping since 2000, delta deposits at the mouth of the Colorado River and tributaries, including Las Vegas Wash, have been eroded by the river flow and redistributed to the deeper parts of the lake.
Sediment cores taken in 1998 have been examined for anthropogenic and natural organic and inorganic contaminants (Covay and Beck 2001; Rosen and Van Metre 2009). In addition, sediment from Las Vegas Wash (the main tributary from Las Vegas) has also been examined for contaminants (Covay and Leiker 1998). These studies found numerous organic compounds associated with urban runoff, industrial contaminants from erosion of the Basic Management Incorporated (BMI) site on Las Vegas Wash, and compounds associated with tertiary treated wastewater effluent, although few compounds were greater than Canadian sediment quality guidelines (Rosen and Van Metre 2009).
In contrast to Lake Mead, remarkably little sediment has accumulated in Lake Mohave since its impoundment in 1953 (Foster 2004). Lake Powell (within Glen Canyon National Recreation Area) and other upstream reservoirs trap virtually all of the sediment transported by the Colorado River. The small amount of fine-grained sediment, which has accumulated, tends to occur in the deepest parts of the lake within sheltered areas along the edges of the drowned
Strategic Fundamental Objectives
Management questions best answered by monitoring:
What is the status and trend of contaminants in sediments? (See also Category 3). more
How does sediment distribution affect spawning potential and reproductive success of sensitive species? (See also Category 2).
How effective are the new Las Vegas Wash wetlands in keeping contaminants out of Las Vegas Bay (see also Category 3. Stressors)?
What is the effect of dredging or other maintenance activities on contaminant release from sediment? (See also Category 3).
Management questions best answered by research:
What happens (e.g., microbial degradation, transport, compaction etc.) to contaminants in sediment?
Are quagga mussels and other invasive species affecting sediment dynamics or vice versa? (See also Category 3).
How do sediments and contaminants interact with the lower part of the food web? (See also Category 2).
What is the role of sediment transport in relation to native fish spawning and fish habitat (see also Category 2. Fish and Aquatic Biota)?
Where do sediments accumulate in greatest abundance? Are there places where contaminants accumulate? What is the fate and transport of contaminants? Is there a subsurface barrier?
What is the relationship between suspended sediment and the aquatic food web and success of razorback sucker (see also Category 2. Fish and Aquatic Biota)? How does turbidity vary seasonally?
What pathogen/sediment associations are present and what causes pathogen remobilization into the water column?
How are the inputs of metals changed over time and place by anthropogenic (e.g., dams, manganese mining), atmospheric (climate change), or other natural actions?
How do wastewater delivery systems impact sediments?