Magnitude, Frequency, and Duration Relations for Suspended Sediment in Stable (“Reference”) Southeastern Streams¹

Abstract

Sediment is listed as one of the leading causes of water‐quality impairments in surface waters of the United States (U.S.). A water body becomes listed by a State, Territory or Tribe if its designated use is not being attained (i.e., impaired). In many cases, the prescribed designated use is aquatic health or habitat, indicating that total maximum daily loads (TMDL) targets for sediment should be functionally related to this use. TMDL targets for sediment transport have been developed for many ecoregions over the past several years using suspended‐sediment yield as a metric. Target values were based on data from “reference” streams or reaches, defined as those exhibiting geomorphic characteristics of equilibrium. This approach has proved useful to some states developing TMDLs for suspended sediment, although one cannot conclude that if a stream exceeds the target range, the aquatic ecosystem will be adversely impacted. To address this problem, historical flow‐transport and sediment‐transport data from hundreds of sites in the Southeastern U.S. were re‐examined to develop parameters (metrics) such as frequency and duration of sediment concentrations. Sites determined as geomorphically stable from field evaluations and from analysis of gauging‐station records were sorted by ecoregion. Mean‐daily flow data obtained from the U.S. Geological Survey were applied to sediment‐transport rating relations to determine suspended‐sediment load for each day of record. The frequency and duration that a given concentration was equaled or exceeded were then calculated to produce a frequency distribution for each site. “Reference” distributions were created using the stable sites in each ecoregion by averaging all of the distributions at specified exceedance intervals. As with the “reference” suspended‐sediment yields, there is a broad range of frequency and duration distributions that reflects the hydrologic and sediment‐transport regimes of the ecoregions. Ecoregions such as the Mississippi Valley Loess Plains (#74) maintain high suspended sediment concentrations for extended periods, whereas coastal plain ecoregions (#63 and 75) show much lower concentrations.

Controls on Suspended Sediment Concentrations and Turbidity within a Reforested, Southern Appalachian Headwater Basin

“…During these storms the reference value was exceeded for a total of less than 10 h on average ( Figure 5). In addition, the maximum measured concentration of suspended sediments (about 140 mg/L) was well below a maximum of approximately 400 mg/L reported on the plots presented by Simon and Klimetz [44]. In summary, then, suspended sediment loads appear to be relatively low within the Allen Creek watershed and are below those that have been considered to be reference values for other stable streams in the area.…”

“…An alternative means of assessing the duration for which biota are subjected to a specific SSC is to determine the percentage of time (frequency) that a given concentration occurs. As pointed out by Simon et al [43] and Simon and Klimetz [44], the methods for documenting such a frequency metric are well established and based on the relationship between flow magnitude and sediment concentration (i.e., the establishment of a sediment rating curve). Within the Allen Creek watershed statistically significant sediment rating curves could be developed for individual runoff events at the three monitored sites (Figure 6a,b).…”

“…As mentioned earlier, the lack of long-term runoff data and an inability to develop SSC vs. discharge relationships inhibited the application of a similar approach to the Allen Creek data. However, it is worth noting that the reference concentration for the 99th percentile storm event (those with flows that are exceeded 1% of the time, or 3.65 days per year) was determined by Simon and Klimetz [44] to be 57.1 mg/L for the Blue Ridge ecoregion. SSC exceeded this 57.1 mg/L reference value at the Allen Creek #1 monitoring site during 11 of the 69 floods recorded between March 2007 and July 2010, or about 3 times per year on average.…”

“…In fact, the reduction appears to be such that SSC is on par with, or below, those suggested as "reference" values for stable streams in the Blue Ridge ecoregion as defined by Simon and Klimetz [44]. The reference concentration was defined as the concentration associated with floods of a specific magnitude and frequency, and was based on regional gaging station data.…”

Use of fish functional traits to associate in-stream suspended sediment transport metrics with biological impairment

“…Channels with scores between 10 and 20 are interpreted as stable or unstable based on their stage of channel evolution. The RGA methodology is described by Simon and Klimetz (2008), and can be referred to for details. RGA for the ER43 study sites were conducted in 2007, and site scores are summarized in ESM 1.…”