Morphologic Response of a River Channel to Sand Mining in River Tyaa,Kitui County,Kenya

Over 40 billion tons of sand is mined worldwide every year which isestimated to be higher than the natural replacement rates. In Kenya, therate of sand mining is raising concerns over its environmental effects sinceit is not regulated. This paper presents findings on the geomorphic effectsof sand mining in the ephemeral River Tyaa channel in Kitui County. Thestudy adopts the concept of feedback response mechanism of a naturalgeomorphic system. Through purposive sampling River Tyaa was selectedfor the study, where rampant sand mining was reportedly taking place.Random sampling on the five sand mining sites identified came up with arepresentative site namely Kanginga on which systematic sampling wasapplied while collecting data at both the active and control sites. Dataon channel width, depth and slope angles was obtained through physicalmeasurements while data on quantity of sand mined was obtained fromMwingi Sand Mining Cooperative. Multiple logistic regression analysiswas used to analyse data whereby the model compared active and controlsites. Test results indicated that sand mining had significantly increasedriver channel’s width (O.R. =1.531), depth (O.R. =1.527) and slope angles(O.R. =1.634) at active mining sites compared to control sites as deducedfrom the respective Odds Ratios. It concluded that sand mining had alteredchannel’s morphology resulting to adverse environmental effects such asloss of riparian vegetation and channel incision. It recommended curbingof illegal sand mining through licencing operators and reducing quantity ofsand mined by closing some mines. Furthers, it recommended monitoringthrough regular Environmental Impact Assessment (E.I.A) and Audit (E.A)to inform protection of the river system from degrading.

The effects of water depth on the growth and reproduction of Potamogeton crispus in an in situ experiment

Aims Water level is one of the most important determinants of the distri-bution and composition of submersed aquatic vegetation in shal-low lakes.Without changes in nutrient loading,large-scale declines in submerged macrophytes in shallow lakes are largely attributed to strong external or internal forces,including changes in water level.As a winter-active submerged macrophyte,Potamogeton crispus has important functions in aquatic ecosystem.The objec-tives of our study were to reveal the effects of water depths on the germination,growth,reproduction and morphological changes of P.crispus under natural environmental conditions;identify the optimum water depth range for colonization of P.crispus;and pre-dict the succession course for P.crispus in Lake Taihu.Methods A long-term in situ experiment was performed to study the effects of water depth on the growth and morphology of P.crispus across the entire life cycle.Plants were grown in the improved cross type of rhizotrons for 30 weeks at three different water depths(60,120 and 200 cm)in Dongtaihu Bay of Lake Taihu.We measured the plant height,root length,the length and width of leaves and counted the numbers of leaves,ramets,turion spikes and roots of each plant on each observation day during the experiment.Important Findings Water depths ranging from 60 to 120 cm were favourable for above-ground vegetation growth,root tissue growth and the reproductive ability of P.crispus.At water depth of 200 cm or beyond,the distri-bution of P.crispus will be limited in the following year because of the lack of turion formation and the severe inhibition of ramet pro-duction.The relationship between above-ground biomass and growth time at three different water depths fits a logistic growth curve well(P<0.001).The rapid growth and distribution with high density of P.crispus at water depth near 60 cm during the reproductive stage is not favourable for growth of other submerged species.So a continuous water depth of~60-120 cm is the optimum growth depth range for the conservation and restoration of P.crispus in Lake Taihu.In addi-tion,the morphological parameters values acquired can replace or provide the ranges for the value of GmaxSB(the maximum growth rate of submerged plants,in the unit of d^(−1))obtained via calibration in the control equations of submerged plant growth in the Eco-Taihu model,which is a three-dimensional ecological model of Lake Taihu.