Assessing the impact of weir construction on recent sedimentation using cesium-137.

1. Harry Williams, Geomorphology

2. Assessing the impact of weir construction on recent sedimentation using cesium-137. Cesium-137 is a man-made radioactive element globally dispersed as fallout since the onset of atmospheric nuclear weapons testing in the 1950's. The shape of the cesium-137 profile (cesium-137 concentration versus sediment depth) can be used to identify two marker horizons: 1954, the base of cesium-137 activity, corresponding to the onset Core 9; from Mad Island Lake. of atmospheric nuclear weapons testing; and, 1963, the peak of cesium-137 deposition after which atmospheric fallout declined due to the Nuclear Test Ban Treaty of 1963. Harry Williams, Geomorphology

3. Cesium-137 released into atmosphere by above ground atomic blasts. Harry Williams, Geomorphology

4. Harry Williams, Geomorphology

5. Mad Island Lake is a small fresh to brackish-water lake located on the shores of Matagorda Bay, Texas. Harry Williams, Geomorphology

6. Mad Island Slough and Northeast Slough are the two largest streams entering Mad Island Lake. Mad Island Slough, a former extension of the west arm of the lake, was impounded by a weir constructed in 1948. Harry Williams, Geomorphology

7. An understanding of sediment supply and sedimentation within the lake is important for management of marsh and lake habitats. Sedimentation at the lake periphery, for example, is required for the maintenance and growth of marsh habitats; sedimentation on the lake bed may reduce the lake's storage capacity, affect water levels in and around the lake, and result in the loss of aquatic habitats. Sediment supply is likely to respond to changes in land use within the lake's watershed, much of which is beyond the boundaries and control of the preserve. Mad Island Slough and Northeast Slough are probably the major sediment sources for the lake. Harry Williams, Geomorphology

8. Emplacement of the weir across Mad Island Slough caused permanent flooding of a large area upstream and created a marshy wetland. The flooded area has undergone sedimentation due to trapping of sediment inflow by the weir. The objectives of this study were to use cesium-137 dating to assess recent sedimentation behind the weir and the impact of the weir on sedimentation in Mad Island Lake. Harry Williams, Geomorphology

9. Ten cores were obtained from the marsh area upstream of the weir. The cores were obtained by pushing 5 cm diameter PVC tubing as far as possible into the marsh sediments. The tubes were cut lengthwise on a table saw and the intact sediment cores extracted. Visible downcore changes in sediment characteristics were recorded. Each core was divided into 4 cm segments and bagged for cesium-137 analysis. Cores from Mad Island Lake had been collected and analyzed in a previous study. Harry Williams, Geomorphology

10. Unlike the cesium-137 profiles obtained from the bed of Mad Island Lake, none of the 10 cores analyzed for this study has a well-defined cesium-137 peak concentration and therefore the depth to the 1963 marker horizon could not be determined. However, in 9 out of 10 cores, cesium-137 concentrations are relatively high in near-surface sediments and fall to zero or near zero values at depth, enabling the 1954 base to cesium-137 activity to be identified. Harry Williams, Geomorphology

11. Initially, the PVC tubes penetrated the sediments fairly easily, but in each case coring was stopped when resistance was encountered at relatively shallow depth. Examination of the cores indicated that the increase in resistance corresponded to a change from soft dark organic-rich mud to more compact less organic-rich sandy deposits at depths ranging from 18 to 36 cm. B horizon? Harry Williams, Geomorphology

12. Core 18 contained a 2 cm thick sand layer at a depth of 16-18 cm. The sand layer was enclosed by dark organic-rich mud. TOP BOTTOM Harry Williams, Geomorphology

13. Results. Relatively little sedimentation has occurred since 1954 (a maximum of only about 36 cm). This may explain why the 1963 peak could not be observed in the cesium-137 profiles; the sampling interval (4 cm) may be too coarse to resolve a peak concentration over such a small thickness of sediment. It may also be that some mixing of the sediments has occurred due to bioturbation. However, the 1954 marker horizon is recognizable on 9 of the cesium-137 profiles providing a measurement of the thickness of post-1954 sediment accumulation. Harry Williams, Geomorphology

14. A second marker horizon is based on the core lithology. The abrupt transition from sandy deposits to overlying organic-rich muddy deposits is interpreted as coinciding with construction of the weir in 1948. The position of this transition, below the 1954 marker horizon in each core, is also consistent with this interpretation. The underlying sandy, organic-poor deposits may represent the former floodplain soil of Mad Island Slough. B horizon Harry Williams, Geomorphology

15. An interpretation is also suggested for the sand layer between 16 and 18 cm depth in core 18. This sand may represent flooding by Hurricane Carla in 1961. The position of the sand layer above the 1954 marker horizon is consistent with this interpretation. The absence of other sand layers within this and the other 9 cores, suggests that deposition of the sand was a rare event and that sedimentation occurred only in isolated areas. Harry Williams, Geomorphology

16. - 1994 - 1948 Core lithologic profiles and 1948, 1954 and 1994 horizons. Harry Williams, Geomorphology

17. Sedimentation Rates Sedimentation rates are derived from the 1948, 1954 and 1994 horizons (Table 1): Harry Williams, Geomorphology

18. Two significant findings are: • relatively little sediment has accumulated behind the weir since 1948 (an average of only 27 cm, based on the core data) • sedimentation rates declined substantially from 1948-1954 to 1954-1994 (from an average of 2.4 cm/year to 0.32 cm/year). Possible reasons for the decline in sedimentation include: • less sediment is being eroded from the surface of the Mad Island Slough watershed (presumably due to land use changes). • more sediment is being trapped within the watershed by barriers such as levees and dikes. • less of the sediment flowing through Mad Island Slough is being trapped behind the weir. Harry Williams, Geomorphology

19. Could more sediment be going over the weir and into the lake? No - in the period 1954-1963, sedimentation in the lake averaged about 6.7 cm/yr; during 1963-1993, this rate fell to 0.83 cm/yr. Cores from the east arm of the lake show the same trend of declining sedimentation. So, both systems (the pond and the lake) are showing a reduction in sediment supply. Harry Williams, Geomorphology

20. Sedimentation in the lake wasn’t drastically reduced after 1948 when the weir was constructed. The sediment must be sufficiently fine (mostly silt and clay) and the flow of water sufficiently turbulent, that a lot of sediment is carried over the weir in suspension, rather than settling out in the ponded area upstream. Why has sediment supply declined? It seems likely that land use changes within the lake's watershed are responsible. Either less sediment is being eroded from the surface of the watershed and/or more sediment is being trapped within the watershed. Wide-spread rice production was initiated in this area in the mid-1950's. Rice requires ponded water, levelling of the surface and the construction of levees to hold in water. These practices are likely to reduce sediment supply; levelling, by reducing surface run-off, and levee construction, by trapping sediment within the watershed. Harry Williams, Geomorphology

21. Conclusions 1. Surprisingly little sediment (averaging 27 cm) has accumulated behind the weir since 1948. Sedimentation has been consistently higher in the lake. Clearly, the weir has not prevented a large amount of sediment from entering the lake. This is presumably due to turbulent flow and fine-grained sediment. Much sediment simply passes over the weir in suspension. 2. Both the weir and the lake exhibit the same trend of declining sedimentation in recent decades. This suggests the supply of sediment from the lake's watershed has declined, possibly due to the initiation of rice production in the 1950's. 3. Two significant findings, in regard to management strategies on the preserve, are that land use practices beyond the boundaries of the preserve are the major control on sedimentation within the preserve, and that physical barriers, such as the weir across Mad Island Slough, do not act as efficient sediment traps in this watershed. Harry Williams, Geomorphology