Word-formation in the Russian language allows to form new words from several, that's why among other compound parts of speech there are adjectives formed from nouns or other adjectives - so-called "compound adjectives" (сложные прилагательные) which have a range of distinctive features in writing. Some of them are written in one word, other - with hyphen.
Spelling of compound adjectives usually depends on the spelling of those words from which they were derived. It means that if a compound adjective is derived from a compound noun, which is written in one word, then this adjective will be written in one word, too. For example,
Водопровод – водопроводная вода - water pipe.
Also if at first a word-combination includes two words, one of which depends on another one, then adjective will also be written in one word.
Вечно зеленый – вечнозеленый лес - evergreen - evergreen forest. Железная дорога – железнодорожный проезд - railroad - railway train.
We should write adjectives derived from terms in one word, too.
Древнеанглийский язык - Old English. Индоевропейская семья - Indo-European family.
We use the second option (with a hyphen) for compound adjectives which are formed from nouns written with hyphen.
We also use with hyphen adjectives formed from words with equal meanings.
Англо-русский словарь - English-Russian dictionary. Светло-голубой цвет - light-blue color. Мясо-рыбный цех - meat and fish shop.
Compound qualitative adjectives are written in the same way (when one quantity adds the notional meaning to another). Кисло-сладкий соус - Sweet n' Sour Sauce.
Geographical names, which include the names of parts of the world (south, north, west, east), are also written with a hyphen. Южно-Китайское море - South China Sea. Северо-западный регион - The north-west region.