Paul F. Hudson

Benthic macroinvertebrates are indicators of water quality and riparian ecosystem health. Their abundance and distribution in streams are associated with river basin environmental factors, including stream biotic and abiotic fluvial processes. We examined benthic macroinvertebrate diversity and abundance in relation to large wood (LW) and pool‐riffle morphology along the Geul River (NL), a small meandering river typical of European landscapes. Restoration since 1998 that includes remeandering and beaver ( Castor fiber ) reintroduction (2002 to 2004) has enhanced the riparian ecosystem. Field sampling was designed to determine abundance and taxa composition of benthic macroinvertebrates, differentiating between sites with LW and without (control) and between pools and riffles. Additional variables included velocity, depth, bed material and canopy cover. Macroinvertebrate diversity to measured environmental factors was evaluated with alpha and beta diversity indices and diversity t ‐tests. Relationships between LW and environmental variables on community composition were assessed using nonmetric multidimensional scaling (NMDS) and analysis of similarities (ANOSIM). Thirty‐one macroinvertebrate taxa were identified, most at the Genus and Family level. Gammaridae (freshwater shrimp) was dominant across all sites. Macroinvertebrate diversity was significantly higher in riffles than pools, while taxa richness was higher for LW in comparison to control sites. Invertebrate community composition was not significantly affected by LW or measured environmental variables. Despite stream restoration, the index of Ephemeroptera, Plecoptera and Trichoptera (EPT) taxa amounted to 33%, indicating degraded water quality. At a basin scale management authorities should strive to reduce agricultural runoff, especially silt inputs. At the channel‐reach scale the pool‐riffle morphology and design of effective LW structures should be a guiding management goal for attaining riparian health consistent with targets of the EU Water Framework Directive.

Lower Mississippi River - Overbank water column (2017-20)

Because most flood sedimentation studies are focused on discreet events it remains challenging to distinguish the importance of different processes on overbank sedimentation, particularly flood duration relative to flood magnitude. We report new field data pertaining to sedimentation thickness (mm) and particle size (mm) for the 2020 hydrologic year along the lower Mississippi River, which extends studies reported from large flood events in 2018-2019 and 2011. This study benefits from having repeat measurements at the same location for different types of flood events to consider flood magnitude relative to flood duration.The study area is a ~25 km long segment of the lower Mississippi alluvial valley between Natchez, Mississippi and Red River Landing, Louisiana, a reach that includes the entire North American drainage of the Mississippi basin which annually undergoes extensive flooding.Flooding in hydrologic year 2020 (at Natchez, MS) occurred from January 15 to June 21, an event of 159 days that is nearly two months longer than the average flood duration. The 2018-2019 compound flood event was overbank a record 286 days. These two sequential events are of much greater duration than the notorious high magnitude events of 2011 and 1973 with a flood duration of 53 days and 90 days, respectively.Particle size of flood deposits from the 2020 event varied most within the first ~200 m of the channel, and then somewhat by depositional setting, including fine-sand (d50 = 0.17 mm) at natural levee crest to fine silt (d50 = 0.011) atop natural levee backslopes, meander scroll -ridge, meander scroll-swale, abandoned channel, and backswamp environments. Despite differences between the 2018-2019 and 2020 events, the average particle size in 2020 is 0.040 mm, somewhat coarser than 0.029 mm of 2018-2019.Recent long duration flood events have finer grained deposits than sedimentary deposits from the record 2011 flood, which averaged 0.049 mm. Additionally, recent long duration flood events (2018-2019, 2020) produced less sand than the 2011 flood, averaging 32% (2011), 14% (2018-2019), and 22% (2020). This points to the importance of flood magnitude in distributing sand across a wider swath of the floodplain, while the high energy event likely flushed fine-sediment downstream and inhibited slackwater sedimentation, which characterized the 2018-2019 and 2020 events.Flood deposit thickness at 41 sites in 2020 averaged 33 mm, notably less than 2018-2019 event (85 mm avg.). Flood deposits from the 2011 event averaged 39 mm in thickness. Sediment thickness should be contextualized against the period over which flood sedimentation occurs. The influence of flood duration results in unit (daily) sedimentation rates for the 2020, 2018-2019, and 2011 flood events being 0.21 mm/day, 0.30 mm/day, and 0.74 mm/day, respectively. Across large lowland floodplains flood duration is more important than flood magnitude to the total amount of sedimentation. Regardless of flood magnitude or duration, a comparison of recent flood sedimentation amounts with the infamous 1973 flood event reveals the persistent decline in Mississippi sediment loads since dam construction of the mid-1900s. Study results are further contextualized by considering (upper basin) sediment province and event-based discharge – suspended sediment dynamics.

Field data are reported for overbank sedimentation generated by the extreme flood event of summer 2021 along the Maas River, an intensively managed lowland river in the Netherlands. Flood duration was short (4 days) but flood magnitude was extreme, the highest stage and discharge recorded in more than 100 years (at Eijsden, NL).Flood sediments were sampled at 108 sites from the NL-BE border to the delta (198-km distance) over a range of hydro-geomorphic environments. Flood deposits were sampled in August and September using conventional field sampling procedures, which included identifying recent sediment deposited atop buried soil and organic layers using field texture and density, and differences in soil color. Sedimentation thickness (mm) of each of the 108 reported values is an average of three individual thickness measurements obtained within a ~1 m radius at each field site. Minimum flood water height was measured by identifying silt and debris lines in vegetation and fencing at multiple locations. The particle size of 77 sediment samples was determined by standard hydrometer analysis and wet sieving to determine sediment indices (D10, D16, D50, D84, D90) to characterise particle size and sediment sorting.Average flood deposit thickness was 19 mm, and varied from 0-mm to 295-mm across geomorphic settings: low floodplains, high floodplains, channel banks, and flood basins and did not exhibit downstream fining. Maximum sedimentation was associated with discreet sand sheets (295 mm). Floodplain stripping (erosion) at some low floodplain sites included reworking and deposition of large clasts (gravel, cobble). Pronounced lateral decreases in sedimentation thickness persists despite flood water height, and rapidly declines beyond about ~30 m from the channel bank. Lateral changes in particle size, however, are less abrupt, and along some reaches very fine sand was deposited to the distal margins of the floodplain. Some laterally distant sites > ~200 m from the channel bank underwent high amounts of sedimentation (38 mm, 25 mm, 43 mm) with pronounced vertical fining (very fine sand to silt) of flood deposits associated with slackwater sedimentation within basins engineered for the Room for the River flood management program. In contrast to many prior sedimentation studies a pattern of downstream fining does not exist, likely due to storage of silts in upstream reaches and downstream high stream power being able to rework of older channel bed deposits.The thickness of the 2021 flood deposits are less than reported for large flood events in 1993 and 1995. This may be due to the shorter duration of the 2021 flood event, as well as the persistent decline in Maas River sediment loads since about the early 1950s, as well as differences in sampling strategy.

Although the Lower Mississippi River has been intensively studied, few studies have investigated meandering processes in the river when the channel was considered to be relatively free of major human disturbance. This research is significant in that it provides a detailed examination of meandering processes and channel adjustment in a large fine-grained alluvial setting, offering a departure from the base of knowledge on this topic that has evolved from studies on smaller coarse-grained river systems. The study spans the entire length of the Lower Mississippi River (1,704 km), from Cairo, IL to Head of Passes, LA. Two sets of hydrographic surveys (scale, 1:20,000) completed between 1877 and 1922 provide the primary source of data. Each survey was segmented into individual meander bends and straight reaches, digitized, and entered into a GIS for analysis of channel processes. Several techniques were employed to examine spatial and temporal change, and interrelationships between channel parameters and controlling factors, including; simple and multiple linear regression analysis, Independent and Paired t-tests, and Pearson correlation coefficients. Two major meandering regimes are identified, which coincide with the boundaries of the alluvial valley and deltaic plain. Channel morphology in the alluvial valley is characterized by its planform morphology having a sinuous laterally migrating channel. Although the cohesive sediments in the deltaic plain reduce lateral migration, the river is able to scour the channel bed into a uniform pool and riffle morphology. Migration rates for channel segments in the alluvial valley and deltaic plain averaged 26.7 and 3.3 m/km/yr, respectively. Analysis of meander bend migration rates with the adjustment of channel curvature between the two surveys suggests an equilibrium meander bend curvature, r$\sb{\rm c}$/W, between 3.0 and 4.0. Spatial trends in sinuosity, channel width, and radius of curvature do not conform to common downstream patterns due to the lack of a downstream trend in discharge. The planform morphology in the alluvial valley increased in sinuosity and decreased in radius of curvature, as meander bends became increasingly arcuate. However, channel width and the pool and riffle morphology did not significantly change during the study period.

New field data are reported for overbank sedimentation generated by the extreme flood event of summer 2021 along the Maas River, an intensively managed lowland river in the Netherlands. Flood duration was short (3-4 days) but flood magnitude was extreme, the highest stage and discharge (3,2650 m^3/s) recorded in more than 100 years.Flood sediments were sampled at 108 sites from the NL-BE border to the delta (195-km distance) over a range of depositional environments, including artificial flood basins created for the Room for the River flood management program. Flood deposits were sampled in August and September using conventional field sampling procedures, which included identifying recent sediment deposited atop buried soil and organic layers using field texture and density, and differences in soil color (recorded). The modal Munsell soil color value for flood deposits and the darker underlying soil were brown (2.5 Y 3/2) and light olive brown (2.5 Y 5/3), respectively. Sedimentation thickness (mm) of each of the 108 reported values is an average of three individual thickness measurements obtained within a ~0.5 m radius at each field site. Minimum flood water height was measured by identifying silt and trash lines in vegetation and fencing at multiple locations and ranged from 3.5-m to 0.3-m above low and high floodplain surfaces, respectively. Particle size of 84 flood sediment samples was determined by hydrometer analysis and wet sieving.Average flood deposit thickness was 21 mm, and varied significantly according to geomorphic setting: low floodplains (28 mm), high floodplains (6 mm), channel banks (31 mm), inset banks (11 mm), and flood basins (42 mm). Maximum sedimentation was associated with discreet sand sheets (295 mm). Floodplain stripping (erosion) at some low floodplain sites included reworking and deposition of large clasts (gravel, cobble). Pronounced lateral decreases in sedimentation thickness persists despite flood water height, and rapidly declines beyond about ~30 m from the channel bank. Lateral changes in particle size, however, are less abrupt, and along some reaches very fine sand was deposited to the distal margins of the embanked floodplain. Some laterally distant sites > ~200 m from the channel bank underwent high amounts of sedimentation (38 mm, 25 mm, 43 mm) with pronounced vertical fining (very fine sand to silt) of flood deposits associated with slackwater sedimentation within basins engineered for the Room for the River flood management program. In contrast to many prior sedimentation studies a pattern of downstream fining (along same geomorphic surface) does not exist, likely due to high stream power and reworking of older channel bed deposits.The overall thickness of the 2021 flood deposits are considerably less than reported for large flood events in 1993 and 1995. This may be due to the shorter duration of the 2021 flood event, as well as the persistent decline in Maas River sediment loads since about the early 1950s, as well as differences in sampling strategy. Study results are further contextualized by considering corresponding event-based discharge – suspended sediment dynamics as well as sediment province.

Flood sedimentary deposits vary due to upper basin and lower basin controls. In this study we focus on overbank sediment thickness, which over longer periods drives changes to riparian aquatic habitat and floodplain construction. The study setting is a ~25 km long segment of the lower Mississippi alluvial valley, between Natchez, MS and Red River Landing, LA. We report new field data for overbank sedimentation generated by compound flooding over 2018 and 2019 hydrologic years, and compare with sedimentation data from prior large flood events. Overbank conditions in 2018 and 2019 persisted for 286 days (at Natchez, MS). During the 2019 hydrologic year the Mississippi was overbank for a record duration of 216 days, resulting in a much greater duration of overbank sedimentation than the 2011 (53 days) and 1973 (90 days) flood events.The thickness of overbank deposits are reported for 48 field sites across a range of depositional environments typical of large lowland meandering river floodplains. Flood deposits were sampled in October 2019 using conventional field sampling procedures, including sedimentation traps (artificial grass mats installed in October 2017) and recognition of recent sediment deposited atop buried organic layers. The thickness of each reported sample is an average of three measurements obtained at each field site.The average thickness of flood deposit samples over 2018-2019 hydrologic years is 71 mm, with variability according to distance from channel and floodplain depositional environment. Maximum sedimentation was associated with crevasse (750 mm) and sand sheet (1,430 mm) deposition along the crest of natural levees. Sedimentation thickness decreases within ~250 m of the channel, but remains high at a distance of ~3.5 km (30 mm). Beyond the range of sand sheet deposition, overbank deposition is likely influenced by variability in floodplain hydrology and geomorphology across natural levee (181 mm), meander scroll (30 mm), old channel (77 mm), and backswamp (108 mm) environments. High backswamp sedimentation at the study site is likely influenced by historic hydraulic engineering for flood control, which has altered local sedimentation patterns.The 2018-2019 sedimentation data are contextualized by comparison with field data from the record 2011 magnitude flood (peak Q of 65,978 m3/s at Vicksburg, MS, USGS 0728900) and the historic 1973 flood (55,558 m3/s).  Average sediment thickness for the 2011 and 1973 overbank deposits was 42 mm (n=49) and 230 mm (n=31), respectively. The 2018-2019 daily sedimentation rate (0.25 mm/day) is much less than 2011 (0.75 mm/day). Thus, the much thicker sedimentary deposits for the 2018-2019 events suggests the greater importance of flood duration – rather than flood magnitude – to overall floodplain processes and alluvial fill chronologies along lowland rivers. The much lower flood sedimentation rate for 2018-2019 in comparison with 1973 (2.49 mm/day) may reveal the persistent decline in Mississippi suspended sediment loads since the early 1950s. Study results are further contextualized by considering corresponding event-based discharge – suspended sediment dynamics, sediment province, as well as flood hydroclimatology.