NOTES on the HYDROLOGY of LAKE NAIVASHA.• The
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NOTES ON THE HYDROLOGY OF LAKE NAIV ASHA .• By H. L. SIKES, C.B.E., B.A., B.E., M.INST.C.E., F.G.S. The hydrological features of Lake Naivashaare not only of scientific interest but are also of some economic importance. A record of tpe fluctuations of lake level has been maintained with four short breaks since tpe end of 19oB, readings being taken weekly. A b.athy• metrical survey was carried out in 1927 with sufficient precision to enable the volume of water and the area of the lake to be c~culated approximately as the level varies. Evaporation meter readings were started in 1917, but, in the case of those prior to 1920, the method adopted is not regarded as sufficiently reliable to be of value; the records are incomplete for the years 1927 and 1928, but complete records exist for 14 years. The number of rain gauges in the drainage area of the lake, 1,203 square miles, is still regrettably small. Only six pave been established and of these the longest record is·from 1904·' However, a number of gauges outside the drainage area, under climatic conditions which appear to correspond fairly well with one or other pIlrt of the drainage area, provide useful data for co-ordination and check. With the help of these a reasonably accurate knowledge of the rainfall on the drainage or catchment area and on Lake Naivasha itself is thougpt to have been achieved, though, in the case of a region where much of the rainfall is of the instability type, precision is im• possible of attainment. Two of the gauges, situated within half a nile of one another, under apparently identical topographical and ~limatic (onditions occasionally show pronounced d.ivergence owing to local thunderstorms, which cause high precipitation on one, but not )0 the other. The circumstance which is thought sufficient to justify deductions ~rom records, provisional though they must necessarily be in some :ases, is that from April 1st, 1935, to March 31st, 1936, the Public Works Department, in the course of hydrographic survey, maintained l continuous record of the flow to the lake of the Melawa (Morendat), :iilgil .and Karati Rivers which drain most of the catchment .area of :"ake Naivasha. By applying the percentage of the rainfall which was lscertajned to have been discharged from areas presenting similar ~eological and topographical features to the remaining portion of the irainage area, where only short flood channels exist, a sufficiently lccurate assessment of the whole surface discharge to t}Je lake during hat year is believed to have been obtained. Lake Naivasha lies in long. 36. 20' E., lat. o· 45' S. Since ecords were commenced, the elevation of jts surface has varied from 73 6,2I7.7o.-feet above mean sea level on November 17t}l, 1917, to 6,194.66 on February 21st, 1936, a range of 23 feet. The volume of water in the lake has fluctuated from 68,400 million cubic feet (.464 cubic mile) to 22,700 million cubic feet (.154 cubic mile), and the superficial area from 2,390 milljon square feet (80.3 square miles) to l_t640 million square feet (58.8 square miles). Its fluctuations from 1909 to 1935 are shown in Plate I, fig. I, and an outline of probabilities in respect of the major changes during preceding years is indicated by a dotted '!jne. TJte greatest depth of the lake is found on the floor· of an extinct crater, about half a square mile in area. Crescent Island and the adjacent island are parts of the wall of the crater. The depth of water within the crater has varied from about 80 feet in 1915 to 57 feet ;n February, 1936. Outside the Crescent Isl.and crater, the maximum dept.h of the l;ike has fluctuated from about 45 feet in 1917 to 22 feet in February, 1936. It is not intended to deal in this paper with the history of Lake Naivasha in Quaternary times. T.he results of the investigations of Dr. Erik Nilsson of the Swedish Geologkal Expedition are recorded in his" Quatern/lryOlaciations /llld Pluvial Lakes in British E/lst Afr;ca " (Stockholm, 1932) and no new light has been thrown on the geological history of the lake since his surveys. The evidence that the highest level of the lake was about 400 feet above present level during the Second Pluvial, extending over the Gilgil pass in continuity with the en1fl.rged Nakuru-Elmenteita L/lke, is not likely to be assailed as it accords with the conclus;ons of other observers. Dr. Nilsson found five other beaches, demarC/lting rest levels of the lake, between the maximum and present-day levels and considers also that the lake dried up completely during two /lrid periods subsequent to the Second Pluvial. Nor is it intended to deal with the geological features of the drainage area of the l;ike, except in so far /lS considerations of percolation from the lake bottom and run off from the drainage area /lre influenced by them. It may be mentioned, however, that excepting the ancient lake sediments consisting of silts, sands, and diatomites, whiCh occupy only a very small percentage of the drainage area, the solid rocks are all volcan;c in origin and are highly faulted by the meridional Rift ValIer fractures. The lavas constitute a varied assemblage of soda• rhyobtes and soda-trachytes, exce,pt in the Aberdare Range, a part of the western flank of which lies within the drainage area, where a different series, ranging from intermedilite to bask, is found east of the early main fault of the Rift Valley. Lake Naivasha occupies a shallow pan in the lowest part of the cross section of the Rift Valley at the latitude of the lake, the drainage area r;sing above it to some 6,000 feet eastward and 4,000 feet westward. T.he volcanic rocks in its vicinity are the most recent of .all. They exhibit sharply defined craters, tuff cones, fresh lavas and numerous steam vents. Meteorolo• gically the drainage area is partly in the sub-ar;d and partly in the 74 A\I~. flGJ VilI\IAilOttS I~LM!: L!:\'EI. BETWEEN .l,PRIL1>TJ935 ANDMARCH 311T. 1936 Vie 2. tro\(;~AU SIt)IV •••• u:)I<THLY GNN-S ,,, \'OLUll£ (l~ LAKt AAOII ~lINf\.,O" rno •• CARHMtKl (2'1 RAmFAI;.\,. CH 'Ai\!; ) R~ 1'1 LAO, LEva ANt> I,.Q~ FP.Q\i If) £\\\FQRATlGN f~ LMI t2'il't!m. .•.TION FPI:)i,l lAHl !\O'tiil!!i())F"'LLIN LAK( Lhtl. WIING nt nlllOD AI'I\l.l iltl~"'!>O MW;~TQ. fO'fAL$roil TH! ItAR; l.It/oN AAINrAl.40" Tijf CATQlN£NUl\tA Of ~ iAlJON ~FEET OR I~} ~UARl MIL£$:• ~'Ie F;'iT OR 1-'II61.!WLUOIlC\I. n:M 1•••\.0'1' INlQ n£ LillIE FROM.,Ht WCIMNr ~A' ~I MILUON "",,/(U' u;I>l-orr 016·111 JlF THt RAmFAu,QR 'lioHUTlK 0EP11i eN tHe:CA'tOlMi;IoIT ""fA. RAINFALLON IH!:L.'E'· H)'Eff OR)1~4 ll'..L!ON CU,FEEt tVAPllM'tlON tROt.i L,A1\(:- 6're fEU OR IOa11,11LUON CU fEET DE~!lUTIOIi'fJICM MV,HE !;I)TTQt./!-9Z F<I1 OR Is..~ I&Ll,ION C\I.,T. A loll;'" n.<l'" OF4i (USEr"> Cfl 'lIlal$tC peR SQ,UILE, UEAN ilR!:'" Cf L.•.IIE 5U1l""i:-~Usq.r,gLt$. REI)l,ICTION Of '/OLUME Of WATER Il'jLA1\E>'~6fEET ();l. 11!IJ31.11WON ro tEtT. j •.... I, ~l AAlAr:' OiVott[ I.AI\t IN INYlLL.I~ "fLLION c.uamr ~.ff(f 1$11» ~2l) llOOO mil um ll360 m(l:) mo ~tQXJ l<'&O WOO ,~ ~IOO ZllIQ r, ~i!OO 21~~ 397W a>6O ~700 19110 311a) IUD 20100 l1ee i\4ilOO Iego LEvEL or MUD 8£f1l!ltol CI\tSCt:NT iSLA/ill MID w.ltfJNO 21200 ISiO 16300 '4110 iTf ; Illlll li-li+H It IHf·· 16300 mo ... <) '" o .. o O. ! 0> It :; f'lGJ VAAfATIONSOf LEVEL OF LAKE NAIVASHA Ml.;II I,NNUM. E'Iil'P'OAA110/1 rliOU I.AI\! I4lA:S. &11f't(T ~N •••rUluAL MAlNl'ALL ON O/WrW:,E ''101[;'27\'l\$ Nlrt Mu.H"'N~~R"jNFAlL ON lo\~f 2HC"RS I·alf[n o !"" !0 ~It) !'" '1'<AAl.Y tIlAPOf'ATlON tilOIolu!(t tl.'IVA$HA ~<_ I\A#<iF'1lLLON OII.~,"GE AIl£A ~'ALL ON,ttlt LAlIt 52 '" 618108...• ••••()3.180",01......() •• ••.•..<l 01'SeJ'".QO<II'".......••••••••"CI...=.Q2-~In.Q$eJ.~ -1"'4-1ft •••• 1:\0 •• 1:\0 . ,!!•••• =,a .••01 ..<l'" o . = = ... '" 1415843.053.803.432.2328.086.145.!!2.817.656.107454.25!:Ill()236IX!u'Cd01=121.291438933••.• ....... AoIeJ425 IX!AolO"'!:IllS.194'" ., ..=:= ~.,j.S::;:;a~~:;;..<ll>-..<l()oIeJ.081~"CI.S3.63·:;;26,42565,470189...•10,3275,004'Cd4,453,,;6,496~=.nnuala......<.S~~..<l:Ii106,675.•..~i1,003a:i~.S.. .S l!g The a 0 .;.....•~s: Totals... for .S sub-humid...s~~h.s:~:;averageis'a'ai zone,=t:s;a~ethe forptere~predominating; the 40 inch isoby.et crosses RemainderKarati ... eJ ~:rf Means- ~s drainagei'"11>-:area "CI s·~ Gllgilrainfall~ ]is assessed at 32.52 inches or 2.71 feet. fromit.Kinangop1936.the observations~iiThe~i~..~~rainfallPlateaut:.i~·I duringdecreaseswestwardi!the rapidlyperiodacross Aprilfromthe· lake.1st,the1935,Aberdareto MarchRange31st, and Melawa. Plate II and the following schedule set forth the results obtained DISCHARGB INTO ~LAKE N AIV ASHA FROM: ITS DRAIN AGK ABBA . • .Assessed as a.verage of percentages of Gllgll and Karati catchments.