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kogi wani mashigin ruwa ne na dabi'a, yawanci ruwa mai kyau, yana gudana zuwa teku, teku, tafki ko wani kogi. A wasu lokuta, kogi yana kwararowa cikin ƙasa ya bushe a ƙarshen tafiyarsa ba tare da ya isa wani ruwa ba. Ana iya kiran ƙananan koguna ta amfani da sunaye kamar rafi, rafi, rivulet, da rill. Babu wasu ma'anoni a hukumance na jimlar kalmar kogi kamar yadda aka yi amfani da su ga fasalin yanki,[1] ko da yake a wasu ƙasashe ko al'ummomi ana bayyana rafi da girmansa. Yawancin sunaye na ƙananan koguna sun keɓanta da wurin yanki; misalan su ne "gudu" a wasu sassan Amurka, "kone" a Scotland da arewa maso gabashin Ingila, da "beck" a arewacin Ingila. Wani lokaci ana bayyana kogi a matsayin ya fi girma fiye da rafi,[2] amma ba koyaushe ba: harshen ba shi da tabbas.[1]

Rafuka suna cikin yanayin zagayowar ruwa. Ruwa gabaɗaya yana tattarawa a cikin kogin daga hazo ta hanyar magudanar ruwa daga magudanar ruwa da sauran maɓuɓɓugar ruwa kamar cajin ruwa na ƙasa, maɓuɓɓugan ruwa, da sakin ruwan da aka adana a cikin ƙanƙara na halitta da dusar ƙanƙara.

Ana la'akari da koguna sau da yawa manyan siffofi a cikin wuri mai faɗi; duk da haka, a zahiri suna rufe kusan kashi 0.1% na ƙasar a Duniya. An bayyana su a fili kuma suna da mahimmanci ga ɗan adam tun da yawancin biranen mutane da wayewar an gina su a kusa da ruwan ruwan da koguna da koguna suke bayarwa.[1] Galibin manyan biranen duniya suna bakin gabar koguna ne, kamar yadda ake amfani da su, ko kuma ake amfani da su a matsayin tushen ruwa, domin samun abinci, da sufuri, a matsayin iyakoki, a matsayin matakan kariya, a matsayin tushen samar da wutar lantarki. don tuka injina, don wanka, da kuma hanyar zubar da shara. A zamanin kafin masana'antu, manyan koguna sun kasance babban cikas ga zirga-zirgar mutane, kayayyaki, da sojoji a fadin su. Garuruwa sukan bunƙasa a ƴan wuraren da za a iya ketare su. Yawancin manyan garuruwa irin su London suna a mafi ƙasƙanci inda za a iya gadar kogi.[2]