1. Flood Regime

Flood Regime Summaries

Characterizing Hydrologic Variability of the Cosumnes River Floodplain

This study characterizes the hydrologic variability of the lower Cosumnes River by analyzing a 98-year streamflow record (1908 – 2005). We develop a flood regime classification methodology by separating similar water year types and similar flood types based on magnitude, duration, and timing.

Nutrient and food resource fluxing through the river floodplain system: An analysis of flood pulse phases as a control on patch dynamics across a restored floodplain.

The objectives are to quantify the flux of nutrients and food resources across the channel-floodplain boundary while delineating the chemical and hydrological signature of different stages of the flood pulse, and to study the influence of the flood pulse on the spatial distribution of suspended algal biomass across the surface of a restored floodplain.

Documents

Flood Regime - Variability

Characterizing Hydrologic Variability of the Cosumnes River Floodplain

Eric G. Booth

Objectives:

  1. Characterize the hydrologic variability of the lower Cosumnes River by analyzing a 98-year streamflow record (1908 – 2005).
  2. Develop a flood regime classification methodology by separating similar water year types and similar flood types based on magnitude, duration, and timing of flooding.

Background:

The Cosumnes River watershed, located southeast of Sacramento, drains a 1989 km2 area starting at 2300 m in the Sierra Nevada mountain range and draining into the Mokelumne River at an elevation of 2 m above sea level. Water from the Cosumnes River ultimately flows into the San Francisco Bay – Delta. It is one of the few unimpounded rivers flowing from the Sierra Nevada Range into the Central Valley. With the exception of loss of base flow in the summer and fall (Fleckenstein et al. in press), the Cosumnes maintains a relatively unimpaired hydrograph.

Methods:

A continuous daily record of discharge data (Figure 3) for the Cosumnes River at Michigan Bar (MHB) from 1908 – 2005 was acquired from the U.S. Geological Survey’s National Water Information System. The MHB streamflow record was analyzed for stationarity because many surrounding basins in the Sierra Nevada exhibit trends in variables such as center of mass of annual flow and maximum annual flow due to changes in climatic conditions since 1950. Several hydrogeomorphically significant thresholds for flood magnitude and duration were developed in order to classify flood events. 10 flood types were created based upon the thresholds and the frequencies at which they occurred throughout the record were calculated. These flood types were then used to develop distinct water year types and their frequencies were calculated.

Key Findings:

  1. The Cosumnes River floodplain experiences two distinct periods of flooding. The first period, occurring roughly from November to February, is comprised of floods that tend to be flashier and have larger peak flows but sustained flooding is not as common during this period as in the second period. The second period, occurring roughly from March to May, contains smaller peak flows compared to the first period but days of flooding are more abundant. These two distinct periods of the flood season are most likely due to later-season snowmelt contributions and larger shallow groundwater inputs in the second period from sources earlier in the season.
  2. This bi-seasonal effect is also reflected in the difference in mean start date for certain flood types – the flood types with larger flood magnitudes and relatively small durations occur early in the season while flood types with longer flood durations and relatively small magnitudes occur later in the season.
  3. Flood types 2 and 3, which consist of the floods that can transport sand onto the floodplain, occur at least once in approximately two out of every three years and twice in half of the years. The very large magnitude type 3 floods occur at least once in one out of every five years on average. The long duration flood types (L and V) occur at least once in roughly six out of every ten years.
  4. The flood type classification along with the flood statistics determined for each of the 479 flood events on record can also be used to test the potential long-term frequency of certain biological phenomena observed on the lowland Cosumnes River floodplain such as the “productivity pumping” described in Ahearn et al. (in review). Based on historical data, at least one productivity pump flood occurred, on average, in two out of every three years, and at least two effective floods occurred in roughly half of the years.
  5. The water year type classification also has the ability to analyze the frequency of certain ecological phenomena but on an annual time-scale. As an example of this connection, Water Year Type (WYT) 7 contains at least one M3 flood, which will most likely create new bare ground in the form of sand deposits, and substantial late-season flooding. Using the Recruitment Box Model (Amlin and Rood 2002), the combination of new bare ground and late-season flooding provides a very favorable condition for the recruitment of cottonwood trees. However, more research is needed to more acutely describe the ecological differences between water year types.
  6. The distribution of certain water year types throughout the period of record also illuminated the previously mentioned observation of the inconsistency of certain aspects of the streamflow record with a stationary time series. WYT-3, a year with a relatively dry winter but a relatively wet spring, decreased in frequency in the second half of the streamflow record (post-1956). In contrast, WYT-6, a year with a very wet winter but a relatively dry spring, increased in frequency in the second half of the record. These two opposite trends are consistent with the hypothesis of a rising snow-rainfall transition line, leading to larger winter floods and diminishing the later snowmelt-dominated part of the hydrograph due to increased winter and spring air temperatures since the mid-20th century (Stewart et al. 2005).

Recommendations for management & monitoring:

As more complex water resources issues surface in the future, managers need to be informed of the degree of hydrologic variability that aquatic ecosystems critically need for them to continue to provide ecosystem services to humans. Organizing flood events and water years into similar types will allow managers to visualize this variability more effectively. While climate will ultimately drive the frequency at which these important floods occur, as a watershed becomes more regulated the water managers will increasingly become more responsible for maintaining the natural frequencies of specific flood types and water year types. A wide-range of hydrologic events are responsible for maintaining the ecological integrity of aquatic ecosystems by resetting ecological succession during large floods, providing ecological cues, and discouraging the persistence of non-native species that are not adapted to natural conditions (Stewardson and Gippel 2003). By knowing roughly what the natural frequencies of specific flood types and water year types were in the recent past, water managers will be able to more accurately provide these aquatic ecosystems with the variability they require to exist.

Flood Regime - Water Chemistry

Nutrient and food resource fluxing through the river floodplain system: An analysis of flood pulse phases as a control on patch dynamics across a restored floodplain

Dylan S. Ahearn

Objectives

  1. Quantify the flux of nutrients and food resources across the channel-floodplain boundary while delineating the chemical and hydrological signature of different stages of the flood pulse.
  2. Study the influence of the flood pulse on the spatial distribution of suspended algal biomass across the surface of a restored floodplain.

Background

The study site was located 34 km south of Sacramento, CA on the lower Cosumnes River, approximately 5.5 km upstream from the confluence with the Mokelumne River (Fig. 1). In 1997, four breaches were cut into the levee bordering the eastern edge of the channel. Two inlet breaches (Tn and Ts) allow river water to enter the 36 ha restored floodplain during floods. Mean flood transit time across the floodplain in 2005 was approximately 1 day; during which time the floodplain held an average of 90 000 m3 of floodwater. After traversing the floodplain, water egresses through two exit breaches (Te and Tw). Mean discharge across the floodplain in 2005 was 6.09 m3 s-1, while during this same period mean discharge in the main channel (as measured at USGS gage #11335000) was 37.1 m3 s-1.

Methods

Between December 2004 and June 2005, samples were collected with ISCO 6700 autosamplers at Tn, Te, and Tw. Water Quality at Ts was assumed to be identical to Tn chemistry. Samples were analyzed for Chl-a, DOC, TIN, TP, TDS, TSS, and VSS. Additionally, a YSI 6600 multiparameter sonde (temperature, pH, TDS, Chl-a, turbidity) with GPS capabilities was employed to collected data at an average of 120 sites across the surface of the floodplain. The autosamplers collected samples during flooding events on a 2-hour time-step. During the 172 days of flooding, 175 samples (169 matched pairs) were collected from each of the three sites. During this same period monitoring of the surface waters of the floodplain was conducted on 22 separate occasions.

Key Findings

  1. Our analysis indicates that periodic connection and disconnection of the floodplain with the channel is vital to the functioning of the floodplain as a source of concentrated suspended algal biomass for downstream aquatic ecosystems.
  2. Peak Chl-a levels on the floodplain occurred during disconnection, reaching levels as high as 25 µ?g l-1. Chlorophyll-a distribution across the floodplain was controlled by water age and local physical/biological conditions, the latter of which were primarily a function of water depth.
  3. During connection, the primary pond on the floodplain exhibited low Chl-a (mean = 3.6??g µl-1) and the shallow littoral zones had elevated concentrations (mean = 5.2 µ??g l-1); during disconnection, the shallow zones Chl-a increased (mean = 11.2 µ??g l-1), but the pond experienced the greatest algal growth (mean = 14.2 µ??g l-1).
  4. Storm-induced floodwaters entering the floodplain not only displaced antecedent floodplain waters, but also redistributed floodplain resources, creating complex mixing dynamics between parcels of water with distinct chemistries. Incomplete replacement of antecedent floodplain waters led to localized hypoxia in non-flushed areas.
  5. The floodplain was an annual sink for all constituents measured (total suspended sediment (TSS): 372 Mt ha-1 yr-1, volatile suspended sediments (VSS): Mt ha-1 yr-1, total inorganic nitrogen (TIN): 0.43 Mt ha-1 yr-1, DOC: 3 Mt ha-1 yr-1, and Chl-a: 0.01 Mt ha-1 yr-1) but closer analysis revealed that some small flooding events caused net DOC and Chl-a export from the floodplain.
  6. Partitioning of the phases of the flood pulse revealed three physically and chemically distinct stages: the flushing phase, the transport phase, and the draining phase. The flushing phase was a brief period of export on the rising limb of the flood, this phase only occurred after an extended period with no upstream connection. The transport phase dominated the flux balance of the system and was marked by retention of all the measured constituents on the floodplain. The draining phase began when outflow from the floodplain exceeded inflow, this phase was an export phase for both DOC and Chl-a.
  7. The fact that small floods were not dominated by the retentive transport phase helps explains why these floods tended to cause net export, rather than retention, of materials on the floodplain.
  8. We propose the notion of “floodplain proportional flooding” for restored floodplain systems, whereby flood size should not overwhelm floodplain volume. In this way residence time is increased as is the potential for the floodplain to be a source for DOC and phytoplankton, valuable food resources for downstream aquatic ecosystems.
  9. The degree of complexity revealed in this analysis makes clear the need for high-resolution spatial and temporal studies such as this to begin to understand the functioning of dynamic and heterogeneous floodplain ecosystems.

Recommendations for management & monitoring

If a primary goal of future floodplain restoration is to create an additional source of food resources for downstream aquatic systems then we recommend “floodplain proportional flooding”, whereby the median flood size does not overwhelm the capacity of the floodplain. Such flooding will assure high MRT on the floodplain for at least one half of the annual floods and thus increase export of DOC and phytoplankton. Given this we must also note the importance of large floods which can export large woody debris and coarse particulate organic matter from the floodplain to the channel, these are also important avenues for energy transfer across the river-floodplain system. Previous studies that have monitored the flux of materials across the channel floodplain boundary have used a relatively coarse sampling strategy. This study has shown that there exist biogeochemically distinct phases to the flood pulse and heterogeneous algal biomass distribution across the floodplain which can not be quantified without high resolution sampling. As such, we recommend intensive sampling in both space and time in order to characterize dynamic and heterogeneous floodplain ecosystem.

Flood Regime Characterization (FRC) for Matlab

This program is an attempt to transform the methods discussed in the companion paper into a portable computer program so that they can be used by researchers and managers on other watersheds in order to characterize a flood regime. The main inputs to the program are a daily streamflow record and a set of flood duration and magnitude thresholds that have some meaningful connection to the hydrologic, geomorphic, and/or ecological processes in the specific watershed. The main outputs are a list of floods, flood types, and water year types along with useful statistics for each list and estimates of the frequency of each flood type and water year type.

Downloads & Resources

  • Program manual (PDF 300KB)
  • Program source code (ZIP 20KB)
  • Example output for Cosumnes River (ZIP 250KB)
  • Link to companion paper (Link)