Hydrography [ Hydrogeologie ]
The river network of the Giant Mts. has its origin in the Tertiary and Quaternary periods and the character of the relief of the mountains is closely connected to its creation. Most of the main Krkonoše streams have their basic direction perpendicular to the main ridge and on the Czech side many of side tributaries empty into them. This is difference from the Polish streams, where due to the steep and sheer slopes with geologic uniformity the rivers are not so branched. The Krkonoše streams have the character of mountain torrents with a steep grade, heavy flow and considerable variation of water level and flows. The river valleys are mostly narrow, closed and in the mountain regions densily forested. The entire region of the Giant Mts., with the exeption of Janské Lázně has no sources of underground water and is exclusively dependent upon precipitation.
Labe (Elbe), the biggest Czech river springs in the Giant Mts. Its source is on Labská louka in the western Giant Mts., 1,386.3 meters a. s. l. Labe drains about one third of the Czech part of the mountains. Among its tributaries in the mountains belong Medvědí potok (Bear's Brook), Bílé Labe (White Labe), and Dolský potok. Malé Labe (Small Labe) and Čistá (Clean) drain the middle part of the Giant Mts., and the eastern part is drained by the Úpa river with Malá Úpa (Small Úpa) and Lysečinský potok (Lysečiny Brook). The western part is drained by the Jizera river with its tributaries Mumlava, Huťský potok and Jizerka.
The river network on the Polish side of the Giant Mts. is simpler and includes the rivers Kamienna, Wrzosówka, Podgórna, Lomnica, and Jedlica. At the same time, there is a water-shed between the Labe and Odra river system along Hraniční hřbet. The water from the Polish slopes of the Giant Mts. flows away to the Baltic Sea whereas from the Czech part of the mountains to the North Sea.
There are a lot of waterfalls on the main Krkonoše streams many of which belong to its most visited attractions. Pančavský vodopád (waterfall) - (140 meters high), Labský (45 m) and Horní Úpský (120 m) fall down from the sheer edges of glacial cirques. Pudlavský vodopád (90 m) and the waterfall of Dvorský potok (40 m) are hidden in forests. Many other waterfalls sprang in places where bedrocks occurred with various hardnesses (Huťský vodopád, falls on Jelení potok (Deer's Brook), cascades on Kotelský and Klínový potok, and in Labská soutěska (gulch)) or their origin was influenced by the tectonic conditions (waterfalls and cascades on Bílé Labe, Kamenica or the well-known fall on the Mumlava river).
The heavy flow and whirling water in the narrow defiles and rapids resulted in bowl-shaped evorsion (potholes). There are over 500 such evorsions on Krkonoše streams (Jizera, Jizerka, Mumlava, Labe, Úpa, Klínový potok, etc.) and so the Giant Mts. take one of the leading positions among streams with such geomorphologic specificity.
Due to glacial modelling authentic glacial lakes arose, mostly in the northern part of the mountains. There are Wielki and Maly Staw, the walled up lakes on the bottoms of the glacial cirques of the same names in the eastern part, and in the west there are tarns on the bottom of Sněžné jámy. There is only one tarn fenced by a moraine - in the valley of Kotelský potok under Kotelní jámy, called Mechové jezírko (Moss Lake). All the other water surfaces are of artificial origin (ponds along the foothills of the Giant Mts.) or they are peat pools and lakes having nothing common with the lakes of glacial origin in respect to their development (lakes on Pančavské, Úpské or Černohorské rašeliniště (peatbogs)).
The Giant Mts. are a significant source region which, by the quantity of the surface water, influence the conditions not only in the mountains themselves, but also in the midpart of the Labe stream in the Czech Republic. Rainfall averages 1600 mm annually in the highest positions annualy most of the precipitation flows away from the mountains in brooks and rivers. It depends on the retaining capability of the mountain slopes and its vegetation, and if the flow-away is continuous and equable for floods to occur. There were some periods in the history of the Giant Mts. during the last century (1882, 1897) and also in this century when the flows exceeded the values of hundred-year-tables and caused enormous damage of property and loss of human lives. However, our forefathers are not blameless in this because they reduced forests on mountain slopes rapidly by destructive forest exploitation and farming, and so affected the hydrological conditions of the mountains.