How do sediment concentrations and chemical compositions change from baseflow to storm events?
Concentrations of sediments in a stream may change dramatically, from quiet baseflow to major storms. Much of the sediment transported by a stream occurs during the relatively small period of time of storms. Hence it is critical to have knowledge of these patterns. Joint with CESAN, the decision was made to initiate 3 field campaigns in the SMV basin, to make field measurements of sediment movement and its chemical composition (because chemistry helps identify where in the basin material comes from). The concentration of suspended sediments, particulate organic carbon (POC), and particulate organic nitrogen (PON) was measured during a week-long period with high rainfall in September 2013 and October/November 2013 and in October 2014 to determine constituent concentrations during peak discharge and assess the response of particulate components to rapid rainfall.
Samples were collected with ISCO autosamplers. Each ISCO sequence collected 24 samples at 3-6h intervals in acid-washed/sample-rinsed 1L HDPE bottles. The ISCO autosampler was filled with ice to minimize degradation of samples throughout the sequence. Samples were collected at approximately mid depth in the center of the stream channels. The isotopic composition of POC and PON was measured throughout the week-long period to assess variability in the sources of organic matter inputs and determine whether specific landscape features were functionalized differently throughout storms and dry periods. Samples were collected on a 3-6h interval in the Rio Santa Maria da Vitoria mainstem and one of its tributaries, the Rio Mangaraí.
In November 2013 sediment measurements were made at a 3-6h interval during a week long period with moderate rainfall to assess sediment concentrations during peak storm flow. It should be noted that river discharge values for this time period are currently unavailbale from the Brazilian monitoring agencies (e.g. ANA), but will be made available once properly quality controlled. Further, once calibrated/validated outputs from the modeling efforts described here can be used to estimate discharge during this sampling period.
From October 30-November 5, 2013 there was periodic light rainfall. Sediment concentrations in both the Santa Maria da Vitoria and Rio Mangarai reflected rainfall/river discharge, ranging from roughly 20-60 mg L-1. During this period, sediment concentrations were more variable (i.e. peaked and dropped more often) in the Rio Mangarai than the Santa Maria da Vitoria main channel. The Rio Mangarai is a small second-order stream where river conditions closely reflect a direct connection with the surrounding landscape. The Rio Santa Maria da Vitoria, on the other hand, recieves tributary and direct inputs from a much larger area. Material concentrations in the Santa Maria da Vitoria are, thus, a blend of signals coming from a large landscape with variable weather and landscape conditions. For example, during the October 30-November 5, 2013 time period the Rio Mangarai received localized rainfall that was not consistent across the entire Santa Maria da Vitoria basin. Sediment concentrations in the Santa Maria da Vitoria main channel, thus, reflected both the inputs of tributaries receiving localized rainfall such as the Rio Mangarai and base flow inputs from tributaries across the basin experiencing dry conditions at the time. From November 5-7, 2013 the entire region experienced significant rainfall. During this period, sediment concentrations in the Rio Mangarai increased from roughly 35 to 600 mg L-1 at peak discharge. Sediment concentrations in the Santa Maria da Vitoria increased from roughly 25 to 325 mg L-1. Similar to the period of light localized rainfall the week before, sediment concentrations increased proportionally less in the Santa Maria da Vitoria compared to the Rio Mangarai, illustrating the direct connection between small streams and their surrounding landscape and the integration of varying signals in larger river channels (Vannote et al., 1980).
Suspended sediment concentrations during storm flow. Measurements of total suspended sediment concentrations during light rainfall (left) and heavy storm flow (right) in the Rio Santa Maria da Vitoria and its tributary, Rio Mangarai, from October 30, 2013 to November 6, 2013. Samples were collected on a 3-6h interval.