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Study region: This study was undertaken in the Athi-Sabaki river basin in Kenya in East Africa. Study focus: The study focused on the determination of the influence of streamflow variability on salt fluxes. This involved monitoring of river discharge and river salinity in the period between 2012 and 2018. New hydrological insights: : This study demonstrates that Athi-Sabaki river discharges significant quantity of salt to the sea. There are significant seasonal and inter-annual variations in salt fluxes that are due to variations in river discharge and rainfall in the basin. The relationship between streamflow variations and variations of salinity in the river was inverse with highest salinity concentrations and fluxes occurring during low flow conditions. The river salinity and TDS concentrations decreased with an increase in river discharge due to dilution effect and flushing of salt from the river. The highly polluted sub-basins draining through the City of Nairobi exhibited relatively higher salinity and salt fluxes as compared to non-polluted ones draining rural areas. The total salt flux in the basin ranged between 29 × 103 and 261 × 103 tons year−1. The relatively high salinity and salt fluxes were attributed to the discharge of wastewaters, seepage of groundwater and irrigation return flows. The study calls for water pollution control, sustainable irrigation and landuse practices in the basin. Keywords: Athi-Sabaki river, Streamflow variations, Salinity variations, Water pollution, Salt fluxes, Kenya

This study was undertaken in Kitui South in Kitui County in the south eastern parts of Kenya. The main objective of the study was to develop a spatial model that can be used for selection of suitable sites for sand dam in Kitui South. The study also assessed the spatial variation of sand grain sizes and how these determine the suitability of sites for the sand dams. Data analysis was done using three approaches. The first approach involved the use of the Remote Sensing (RS) technique where data was preprocessed and analyzed using Erdas Imagine software and Geographical Information Systems (GIS) for spatial modelling. The distribution of sand particle-sizes was based on sieve analysis on the sand samples collected from seasonal rivers in the study area. The seasonal rivers that were investigated in this study included Mwila, Kakya, Wiitu, Nguni, Ngunyumu, Muvuko, Kanzilu, Masaa, Katiliku, Ngulungu, Nzeeu, Koma and Katitika. Statistical analysis was done to establish the relationships between the various spatial components affecting sand particle distribution. The results from the spatial model which integrated all factors shows that 16% of the studied sites along the seasonal rivers in Kitui South were fairly suitable for construction of sand dams while 79% were classified as suitable and 5% are categorized as very suitable. Several sites in the central and the entire eastern parts of Mutha Ward were found not suitable for sand dam construction due to poor conditions such as the lack of suitable sand particles, lack of bedrock exposure on the riverbeds, flat terrain, very shallow stream banks, and weak soils on the riverbanks. Suitable sites for sand dams were found on the western and central areas of Kitui South where production and high accumulation of coarse sands was observed along the seasonal rivers. The areas near Mutomo hills and the rocky areas in Ikanga wards were found to be characterized by few excellent sites for sand dams. The optimum accumulation of different grades of sand in the seasonal rivers was found on streams with slope of between 1.5 and 6% that were dominated with 0 to 40% coarse sands, 0 to 30% fine gravels, 0 to 20% fine sands, and 20 to 80% medium sands. The accumulation of fine gravels was found to be closely related to the distribution of D30, D60 and D90 sand particle-sizes (0.4-4 mm) while accumulation of coarse sands showed good relationship with the distribution of the medium size sand (0.2-0.4 mm). The accumulation of medium sands was equally high when the occurrence of coarse sands was minimal. Different grades of sand were found to be strongly influenced by the changes in the elevation of the stream especially with regard to the accumulation of the medium sands (r=0.76) and coarse sands(r=0.75). Out of 80 investigated sites, 59% of them were dominated with uniform sands (Cu 5). The study also found that 86% of the sites along the seasonal streams were dominated with medium sands while 10% were dominated with fine sands and the rest (4%) dominated by coarse and gravelly sands. From the results this study concludes that 59% of the 80 sites that were investigated in this study have a high potential for providing suitable sites for sand dams in Kitui South.

This chapter presents the results of a study on the influence of streamflow variability on salinity, total dissolved solids (TDS) and conductivity in a semi-arid Tiva River Basin in Kenya. Measurements of salinity, TDS, conductivity and river discharges were undertaken in sampling stations by applying standard hydrologic methods. The study shows that there is a significant relationship between the variability of streamflow and the variability of salinity, conductivity and TDS in the river. The high salt concentrations were a result of high evapotranspiration and seepage of subterranean water from bank storage and groundwater aquifers. Inter-sub-basin variations in the levels of salinity were attributed to differences in land uses, lengths and sizes of the sub-basins. Sustainable irrigation and land use practices in the semi arid Tiva River Basin requires construction of water reservoirs for control of salinity levels in the river.

For an improved understanding of the hydrometeorological conditions of the Tana River basin of Kenya, East Africa, its joint atmospheric-terrestrial water balances are investigated. This is achieved through the application of the Weather Research and Forecasting (WRF) and the fully coupled WRF-Hydro modeling system over the Mathioya-Sagana subcatchment (3279 km2) and its surroundings in the upper Tana River basin for 4 years (2011–2014). The model setup consists of an outer domain at 25 km (East Africa) and an inner one at 5-km (Mathioya-Sagana subcatchment) horizontal resolution. The WRF-Hydro inner domain is enhanced with hydrological routing at 500-m horizontal resolution. The results from the fully coupled modeling system are compared to those of the WRF-only model. The coupled WRF-Hydro slightly reduces precipitation, evapotranspiration, and the soil water storage but increases runoff. The total precipitation from March to May and October to December for WRF-only (974 mm/year) and coupled WRF-Hydro (940 mm/year) is closer to that derived from the Climate Hazards Group Infrared Precipitation with Stations (CHIRPS) data (989 mm/year) than from the TRMM (795 mm/year) precipitation product. The coupled WRF-Hydro-accumulated discharge (323 mm/year) is close to that observed (333 mm/year). However, the coupled WRF-Hydro underestimates the observed peak flows registering low but acceptable NSE (0.02) and RSR (0.99) at daily time step. The precipitation recycling and efficiency measures between WRF-only and coupled WRF-Hydro are very close and small. This suggests that most of precipitation in the region comes from moisture advection from the outside of the analysis domain, indicating a minor impact of potential land-precipitation feedback mechanisms in this case. The coupled WRF-Hydro nonetheless serves as a tool in quantifying the atmospheric-terrestrial water balance in this region.

Sediment yield in the Upper Tana Basin in Kenya has implications on the sustainability of Hydro-Electric Power (HEP) dams and water resources development projects. Therefore, a study was undertaken in the basin to establish the extent to which rainfall and river discharges influence the sediment yield in the catchment. The study was based on hydrological data obtained from the Water Resources Management Authority (WARMA) and Kenya Meteorological Department (KMD). The river discharge data was obtained from three RGS Maragua (4BE01), Gura (4AD01) and Tana Sagana (4BC02) and rainfall data was obtained from Sagana Fish Farm and Nyeri Ministry of Works for the of period 1960-2013.The study also applied the Soil Water Assessment Tool (SWAT) Model to determine the extent to which the model can be used to simulate streamflow and sediment yield in the basin. The results of the study showed that there is a significant variability in streamflow and sediment yield in the Upper Tana Basin. In the period between 1960 and 2015, the mean total annual river discharge of Tana Sagana was 128 m3s-1, and the maximum and minimum river discharges were 29.94 m3s-1 and 3.15 m3s-1, respectively. There was an indication of increasing trend in rainfall and subsequently sediment yield in the basin, which may be attributed to alteration of land use and climatic change. The results showed that SWAT model was quite good in simulating the variability of river discharge. The analysis revealed a poor relationship between sediment yield and rainfall. However, the relationship between rainfall and stream flow was strong with r value of 0.9 which is significant at p=0.05. Relationship between simulated and observed river discharge had a R2 of 0.442, r of 0.665 and NSE of -89.43. The relationship between simulated and observed sediment yield had a R2 of 0.733, r of 0.86 and NSE of 0.69. The results of this study showed that SWAT model can be used to predict sediment yield in the Upper Tana catchment. The model had good performance when daily rainfall, stream flow and sediment yield data were used. Thus, the model can be used to establish the relationship between rainfall, discharge, and sediment yield in a highly human-impacted tropical catchment area. The study puts also forward various recommendations on land and water resources management in the basin.

This study is focused on the determination of the extent to which changes in river freshwater and sediment input affects the sustainability of the Tana Delta and Sabaki estuaries in Kenya. The study involved the determination of river freshwater and sediment fluxes, as well as water exchange and sediment fluxes at the mouths of the two estuaries. The horizontal and vertical distributions of tidal current velocities, salinity and total suspended sediment concentrations (TSSC) within the estuaries enabled determination of the degree of stratification and the extent to which mixing of seawater and freshwater leads to the formation of the maximum turbidity zone (TMZ) in the two estuaries. The two estuaries are important for biodiversity conservation, sustainability of socio-economic livelihoods and provision of global environmental benefits. The study shows that the hydrologic dynamics controlling water circulation including the trapping and exchange of terrigenous sediments in the two estuaries is a function of the river discharge and tidal forcing. In the much smaller Sabaki estuary, there has been a reduction in freshwater input and an increase in sediment supply leading to heavy accretion. The shallow nature of the Sabaki estuary ensures reduced penetration of the semi-diurnal tidal wave into the estuary and seawater intrusion is restricted to 2.5 km of the estuary. On the other hand, there has been a substantial reduction in both freshwater input and sediment supply into the Tana Delta. This has led to deepening of the estuary channels with the result that tidal wave penetrates much deeper into the estuary and seawater intrudes up to 10 km inside the estuary. The tidal asymmetry in the two estuaries is characterized by ebb tidal flow dominance due to presence of mangrove forests, wide intertidal areas and freshwater input. This has resulted in net export of sediments out of the two estuaries. However, the cohesive clay sediments are trapped within the estuaries in mangrove forest wetlands and in sheltered intertidal areas that are now occupied by mudflats. The shallow Sabaki estuary experiences greater rates of water and sediment exchange as compared to the relatively deeper Tana estuary. The changes in freshwater and sediment supply into the two estuaries were attributed to landuse change, damming and climatic variability. The major impacts in both estuaries include high turbidity, heavy sedimentation, changes in beach morphology and degradation of the marine ecosystems such as the coral reefs and seagrass beds. In the Tana delta system, the impacts include high turbidity, alteration of the morphology of the delta, degradation of the mangrove forests, coastal erosion and sea water intrusion. The study notes that the proposed large-scale hydropower and irrigation projects in the Athi-Sabaki and Tana river basins have the potential of causing massive degradation of the two estuaries. The paper puts forward recommendations for sustainable management of the two estuaries in Kenya.

Anthropogenic factors associated with damming and water abstraction, and the resultant environmental pressures, are reviewed in six African river catchments using records and forecasts of climatic, demographic, and land-use change. Changes in the states of the flow regime through catchment drainage systems to the coastal sea are considered in conjunction with climate change and other human-induced pressures. The impacts of these changes on downstream and coastal environments and their communities are described in past, present, and future perspectives. Linkages between the issues and the pressures of damming and water abstraction are appraised and scientific, policy, and management responses proposed aimed at remedying existing and perceived future negative impacts. The study proposes that there is a need to integrate catchment and coastal management to account for the whole water flow regime together with its human dimensions. Management priorities relating to the operation of existing damming and abstraction schemes and planning of future schemes include the following: consideration of ways in which water discharges could be adjusted to provide improvements in downstream and coastal environmental and socioeconomic conditions; addressing the problem of sediment trapping impacting on the sustainability of dam reservoirs; and assessment of downstream and coastal impacts of future schemes in the light of climate change forecasts.

This study focuses on sediment transport and exchange dynamics in the 27 km2 Tana Estuary located at Kipini in the north Kenya coast. The estuary is drained by the Tana River, which contributes more than 50% of the total river discharges into the Kenyan sector of the Indian Ocean. The study involved measurement of river discharges, estuarine flood–ebb tidal discharges, total and particulate organic suspended sediment concentrations (TSSC, POSC) and fluxes, tidal water elevation, current velocities, temperature and salinity. The study was conducted between February 2001 and November 2003. The results of the study showed that the Tana River discharge ranged between 60 and 750 m3 s−1. The maximum river discharges during the Southeast and Northeast monsoons were 750 and 350 m3 s−1, respectively. The peak river discharges occurred in May and November. The total daily sediment load of the Tana River varied from 2796 tons day−1 during the dry season to 24,322 tons day−1 during the rainy season. The annual total sediment load is 6.8 × 106 tons yr−1, which is, however, smaller than that before the damming of the river in the Upper Tana Basin. Because of river discharge and occurrence of a wide mangrove wetland, the estuary is mainly ebb tide dominant. The peak ebb tide currents (0.87 m s−1) were 30% larger than the peak flood tide currents (0.65 m s−1). During spring tide, the estuary was partially well mixed. However, in neap tide, the estuary was stratified in the middle zone. The turbidity maximum zone with TSSC > 1.5 g l−1 was generated in spring tide by wave stirring of frontwater zone bed sediments and trapping of fine sediments at the toe of the salt wedge. The peak TSSC ranging from 1.5 to 5.0 g l−1 occurred at low tide due to river supply of sediment and resuspension of channel bed sediments by wind waves, particularly at the last stages of ebb tide and early stages of flood tide. Due to the influx of oceanic water into the estuary at flood tide TSSC decreased to

This study focuses on sediment exchange in the degraded Mwache mangrove forest wetland located in southern Kenya. It involved measurement of total and particulate organic suspended sediment concentrations (TSSC and POSC), tidal water elevation and current velocities. Results showed that in the heavily degraded backwater zone mangrove forest, the ebb and flood tide total sediment fluxes were of same order of magnitude, however, flood tide sediment fluxes were slightly higher than the ebb ones. In the moderately degraded frontwater zone mangrove forest, the flood tide sediment fluxes were more than 50% higher than the ebb tide fluxes. The peak net sedimentation in the highly degraded backwater zone was 4 g m −2 tide −1 but that in the moderately degraded frontwater zone was 63 g m −2 tide −1 . In the frontwater zone of the mangrove forest, the peak instantaneous ebb tide sediment flux was 3206 kg tide −1 equivalent to 35.6 g m −2 tide −1 and the flood one 8574 kg tide −1 (95 g m −2 tide −1 ). The peak instantaneous flood and ebb tide particulate organic sediment (POS) fluxes in the frontwater zone mangrove forest were 1316 kg tide −1 (15 g m −2 tide −1 ) and 587 kg tide −1 (6.5 g m −2 tide −1 ), respectively. The peak ebb and flood tide sediment fluxes in the backwater mangrove forest were 3206 kg tide −1 (36 g m −2 tide −1 ) and 3305 kg tide −1 (36.7 g m −2 tide −1 ), respectively. In case of POS fluxes in the backwater zone mangrove forest, the peak flood period POS flux was 969 kg tide −1 (10.7 g m −2 tide −1 ) while the ebb period one was 484 kg tide −1 (5.4 g m −2 tide −1 ). In both highly degraded backwater and moderately degraded frontwater zone of the mangrove forest, there is net import of sediments. However, the net import is relatively lower in the backwater zone forest where the trapping efficiency is 27%. In the moderately degraded frontwater zone of the mangrove forest, the sediment trapping efficiency is 65%. The net sediment import occurs mainly in periods of high river discharge in both neap and spring tides, but occurs only in spring tides during dry season. The net accretion rates in the backwater and frontwater zone mangrove forests are 0.25 and 3.5 cm year −1 , respectively.

This study focuses on sediment exchange dynamics in Mwache Creek, a shallow tidal mangrove wetland in Kenya. The surface area of the creek is 17 km2 at high water spring. The creek experiences semidiurnal tides with tidal ranges of 3.2 m and 1.4 m during spring and neap tides, respectively. The creek is ebb dominant in the frontwater zone main channel and is flood dominant in the backwater zone main channel. During rainy season, the creek receives freshwater and terrigenous sediments from the seasonal Mwache River. Heavy supply of terrigenous sediments during the El Nino of 1997–1998 led to the huge deposition of sediments (106 tonnes) in the wetland that caused massive destruction of the mangrove forest in the upper region. In this study, sea level, tidal discharges, tidal current velocities, salinity, total suspended sediment concentrations (TSSC) and particulate organic sediment concentrations (POSC) measured in stations established within the main channel and also within the mangrove forests,...

Water circulation and exchange processes in a shallow, semi-enclosed tropical bay were studied in southern Kenya (Gazi Bay) through measurements of tidal elevations, salinity, temperature, dissolved oxygen and current velocities at stations established in mangrove creeks, seagrass beds and coral reef zones. Occurrence of wide shallow entrance, lack of topographic controls (sills) and the orientation of the Bay entrance with respect to dominant tidal water circulation patterns, accounts for the high rates of exchange (60–90% of the volume per tidal cycle) between the inshore and offshore waters. High flushing rates are coupled with short residence times in the order of 3–4 h. The dominant water circulation driving force is the semi-diurnal tide, causing a strong reversing current in the mangrove creeks (0·6 ms−1) and low magnitude current in the seagrass and coral reef zones (

Water circulation, water column nutrients and plankton productivity were studied in a tropical bay with high rates of water exchange (60% to 90% per tide) and short residence times (3 to 4 h). The water circulation is predominantly affected by the semi-diurnal tides, which cause strong and reversing currents in the mangrove creeks (0.60 m·s−1) and currents of low magnitude in the neighbouring seagrass and coral reef zones (

Water circulation patterns in a tropical mangrove-fringed bay with seagrass and coral reef are driven by tides that generate strong reversing tidal currents. The wind, which has an onshore component, generates a net clockwise-rotating eddy. The dominant tidally driven water circulation pattern, coupled with the effects of onshore wind and alongshore current generated by wave breaking, promotes the coastal trapping of turbid brackish water and its inherent nutrient content. This brackish water inundates the mangrove swamp and seagrass beds but not the coral reef ecosystem. Weak stratification prevails during the wet season in the upper parts of Kidogoweni Creek as a result of freshwater influx from rivers. In the dry season, well-mixed homogeneous water is found in most regions of the bay. A small zone of hypersaline water (salinity reaching 38 PSU) is found in the upper region of the mangrove-dominated creeks during the dry season. The connection between the mangrove swamp, with its wide salinity variations, and seagrass beds is apparently through river plumes and tidal effects. The link between seagrass beds and coral reefs is mainly through tidal influences.

Soil and Water Assessment Tool (SWAT) was used to model the impacts of land cover changes on stream flow regime in the Thiba River basin covering a surface area of 1648 km2 in central region of Kenya. The basin is characterized by intensive agricultural activities including the largest rice irrigation scheme in Kenya. A study was undertaken to test the capability of the model in predicting stream flow response under changing land use conditions in a typical tropical river basin. Classified land use maps of 1984, 2004 and 2014 were analyzed to investigate land use changes in the basin. Field based survey, National Irrigation Board (NIB), Kenya Meteorological Department and Water Resources Management Authority (WRMA) provided hydro-meteorological data for the study. The results of the study shows that forest cover in the Thiba River basin has decreased by 18.39 % between 1984 and 2014 while area under rice cultivation increased by 9.38 % in the same period. The SWAT Model results showed that there is a significant relationship between the observed and simulated average monthly stream flows in the Thiba River Basin. The Nash-Sutcliffe Efficiency (NSE) and coefficient of determination (R2) during calibration period (1983-1988) were 0.82 and 0.9, respectively, while for the validation period (1989-1993) they were 0.79 and 0.87, respectively. The average monthly stream flows increased by 6.01 m3/s during the wet season and decreased by 1.92 m3/s during the dry season. The changes in stream flow were attributed to the land cover change and rainfall variability. About 35% of dry season flow and 3% of wet season flow was found to have been directly abstracted from the Thiba River. The study recommends that the basin stakeholders should optimize utilization of abstracted water to avert future catastrophic stream flow fluctuations, possibly flooding during the wet season and low or dry riverbeds during the dry months. The high water demand in the dry months can be met by constructing water storage reservoirs to harvest the high runoff during the wet months. Also, it's important that further research on impact of climate change be conducted to better understand the relationship between catchment hydrology and climate change.

[Extract] There are two dominant types of mangrove swamps, the riverine type that fringes rivers and tidal creeks, and the open water type that is directly exposed by waves (Lugo & Snedaker, 1974). The former type is the most common, with a strip of mangroves typically 50 to 300m wide fringing the tidal creek or river on either side. Such an example is the 5-km-long mangrove-fringed Merbok River estuary in Malaysia (Figure 1). The second type is generally present only in embayments protected by shallow reefs and mud or sand banks that allow wave attack only around high tide. Missionary Bay in Australia (Figure 1b) is a typical example of an extensive mangrove swamp that is protected from the prevailing trade winds but nevertheless is occasionally attacked by waves in the monsoon season. Along coral reefs, mangroves can also be present and are protected from excessive wave attack by waves breaking on the fringing reefs. Along muddy coasts a strip of mangroves, typically a few hundred meters wide, can fringe the open coast, and these are very frequently under wave attack; nevertheless, they survive. Such is the case of the Thuy Hai coast of the Gulf of Tonkin in Vietnam (Figure 1c).