Kogin
kogin wani mashigin ruwa ne na dabi'a, yawanci rafi ne mai ruwa mai dadi, yana gudana a sama ko cikin kogo zuwa wani ruwa a kasa mai tsayi kamar teku, teku, bay, tabki, dausayi, ko wani kogi. A wasu lokuta, kogi yana gudana cikin ƙasa ko ya bushe a ƙarshen tafiyarsa ba tare da isa wani ruwa ba. Ana iya kiran ƙananan koguna da sunaye kamar rafi, rafi, da rivulet . Babu wasu ma'anoni a hukumance na jimlar kalmar kogi kamar yadda aka yi amfani da su ga fasalin yanayin ƙasa,  ko da yake a wasu ƙasashe ko al'ummomi, ana bayyana rafi da girmansa. Yawancin sunaye na ƙananan koguna sun keɓanta da wurin yanki; Misalai sune "gudu" a wasu sassan Amurka, " kuna " a Scotland da Arewa maso Gabashin Ingila, da "beck" a Arewacin Ingila . Wani lokaci ana siffanta kogin da ya fi kogi girma,  amma ba koyaushe ba; harshen ba shi da tabbasko

Koguna muhimmin bangare ne na zagayowar ruwa . Ruwa daga magudanar ruwa gabaɗaya yana tattarawa cikin kogi ta hanyar ruwan sama daga hazo, ruwan narkewar da ake fitarwa daga kankara da fakitin dusar ƙanƙara, da sauran hanyoyin ƙarƙashin ƙasa kamar ruwa na ƙasa da maɓuɓɓugan ruwa . 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. Har ila yau, koguna ne mai muhimmanci na halitta terraformer, kamar yadda erosive mataki na Gudun ruwa sassaƙa fitar da rills, gullies, da kwaruruka a cikin surface kazalika da canja wurin silt da narkar da ma'adanai a kasa, kafa kogin deltas da tsibirai inda kwarara ya ragu. A matsayin ruwa, koguna kuma suna yin ayyuka masu mahimmanci na muhalli ta hanyar samarwa da ciyar da wuraren zama na ruwa don ruwa da namun ruwa da flora, musamman ga nau'in kifin da ke ƙaura, tare da ba da damar yanayin yanayin ƙasa don bunƙasa a cikin yankunan magudanan ruwa .

Koguna suna da mahimmanci ga ɗan adam tunda yawancin ƙauyuka da wayewar mutane an gina su a kusa da manyan koguna da koguna.  Yawancin manyan biranen duniya suna kan gabar koguna, kamar yadda suke (ko kuma) sun dogara ne a matsayin tushen ruwa mai mahimmanci, don samar da abinci ta hanyar kamun kifi da ban ruwa, don jigilar kaya, azaman iyakoki na halitta da/ ko wuraren kariya, a matsayin tushen samar da wutar lantarki don tuka injina ko samar da wutar lantarki, 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 yankuna. Garuruwa sukan bunƙasa a ƴan wuraren da suka dace da titin mota, gina gadoji, ko tashoshin jiragen ruwa ; manyan biranen da yawa, irin su Landan, suna a mafi ƙanƙanta kuma mafi aminci wuraren da za a iya ketare kogi ta gadoji ko jiragen ruwa . 

A fannonin kimiyyar duniya, ilmin dankalin turawa shine binciken kimiyya na koguna, yayin da ilimin ilimin ilimin kimiyyar ilimin halittu shine nazarin ruwa na cikin ƙasa gabaɗaya.

Tushen da magudanar ruwa 
Kogin yana farawa ne daga wani tushe (ko mafi yawan maɓuɓɓuka da yawa) wanda yawanci magudanar ruwa ne, yana magudanar da duk rafukan da ke cikin magudanar ruwa, ya bi magudanar ruwa, ya ƙare ko dai a baki ko baki wanda zai iya zama haɗuwa, kogin delta, da dai sauransu. . Ruwan da ke cikin kogi yawanci yana iyakance ne a tashar tashoshi, an yi shi da gadon rafi tsakanin bankunan . A cikin manyan koguna, galibi ana samun faffadar ambaliya mai siffa ta hanyar ambaliya sama da tasha. Ambaliyar ruwa na iya zama mai faɗi sosai dangane da girman tashar kogin. Wannan bambance-bambancen da ke tsakanin tashar kogi da filin ambaliya na iya zama duhu, musamman a cikin biranen da filayen kogin na iya bunkasa sosai ta hanyar gidaje da masana'antu.

Ma'anar " harbe " da " ƙasa " suna nufin alkibla zuwa tushen kogin da kuma bakin kogin, bi da bi.

Tashoshi 

Koguna
Shafuka masu fassarorin da ba'a duba ba