The term "low water" comes from the concept of "border", that is, "border". However, it is directly related to hydrology. And if high water indicates an excess of the normal water level, then low water, on the contrary, indicates a decrease. What are the causes of these processes, what consequences can they have, and what role does a person play in all this?
History of the term
We have already mentioned the nature of the origin of the term. Indeed, the state of rivers, and indeed of all nature in general, in ancient times was a kind of boundary and boundary of the transition from one season to another. Therefore, low water was a kind of calendar. Not without reason, after all, all the most ancient civilizations arose in riverbeds. After all, these reservoirs for people have always been not only a source of food, but also a means of communication and information.
Later, the state of the channel began to play a key role in navigation, because for thousands of years all trade and passenger flows were carried out along them. And if you think about it, the low water of the river greatly complicated this movement. That's where they went fromfamous haulers. Entire gangs of peasant men pulled huge barges on ropes, often also loaded with goods. And on the Sura River there were even women's artels of barge haulers.
Why does low water occur
In hydrology, there are several explanations for the occurrence of the low water period. First of all, these are natural reasons for the decrease in water content, when the entire inflow of water occurs exclusively due to groundwater. That is, in such seasons when there is no flood. Traditionally it is winter and summer. Precipitation during these seasons cannot provide the required amount of water, so the channel becomes shallow. Summer low water is especially typical for the southern regions, where small rivers can dry up completely, and vegetation even appears at the bottom of the channel.
The so-called growing season also affects the state of the channel and the occurrence of low water. This is a period of time when all kinds of plants grow actively along the banks and at the bottom of the rivers, and then die off. Too abundant vegetation has a negative effect on the condition of the channel. This is especially true for lowland rivers.
Of course, low water is not only a change of seasons. A lot depends on the average air temperature, as well as the water itself. In dry, hot summers, moisture evaporates very quickly and can lead to a critical decrease in water content. The speed of the current, the depth of the stream and the type of bottom soil can also become decisive.
Dependence of water level on the season
But whatever one may say, seasonality plays a key role for rivers. Most sources agree that low wateris mainly a seasonal phenomenon. The period of the lowest water content of the river averages at least 10 days.
In most temperate regions, low water begins at the end of summer and continues until the onset of freeze-up. An exception may be mountainous areas, where heavy rains often cause floods and a sharp increase in water in rivers.
But it can be difficult to unambiguously determine the timing of low water. Summer low water can stretch from the end of the spring flood to the very beginning of prolonged autumn rains. That is, in fact, for the entire season. And in case of low rainfall, go to the autumn low water. The only season that is certainly not characterized by this phenomenon is spring, when the melting of snow fully compensates for any moisture deficit.
Winter low water
The winter season is a completely different matter, when hydrologists distinguish a special state of the river bed. In a significant part of the continent, a stable freeze-up persists for half a year. The most critical marks of the cold season are November and December, that is, the period of formation of the ice cover. On average, winter low water can last up to 170 days. Smaller streams can freeze through, especially if they flow through karst areas.
During the winter low water, while the river is covered with ice, as well as in the summer-autumn period, in conditions of scarcity of precipitation, it is fed exclusively from groundwater sources.
River low water in different climatic zones
Climaticwatercourse belt. As we have already found out, in a temperate climate they mainly have a mixed diet - rain and snow, as well as underground, prevailing at different times of the year.
And, for example, the rivers of the equatorial belt receive mainly food from precipitation. Here, too, seasonality takes place, and the duration and intensity of the rainy season acquires the greatest importance. There are practically no underground sources here. Whereas in the tropical zone, rivers are filled almost exclusively due to underground feeding.
A typical example of a temperate river is the Volga, which floods in spring and becomes very shallow by the end of summer. Classical equatorial - Amazon, actively filled from December to April, when more than 60% of precipitation falls on this territory.
How is low water determined?
We are entering professional hydrological territory. For a general understanding, the most common criterion, called "change in runoff volume", is sufficient. The time when this volume decreases to 15% of the annual volume can be considered a period of low water. It is noteworthy that science considers the decrease in the water level in the channel to be the most stable. In this regard, on large topographic maps, rivers are depicted exactly as they are during the low water period.
If we talk about flat watercourses of the middle zone, they are characterized by low water levels in summer. And, for example, in the mountains, spring snowmelt stretches for the entireseason and captures the summer when the ice cover begins to melt. Therefore, low-water levels are practically not found in mountain rivers. They always retain a sufficient supply of moisture, including due to heavy and frequent precipitation. In the Far East region, where floods are frequent even in hot weather, summer low water levels are also rarely observed. This is not surprising, because the precipitation there is frequent and plentiful.
Global Processes
Summer and winter low water are not only constantly repeating processes, like the change of seasons. Climatic factors such as prolonged droughts in summer and lack of rainfall in winter can cause more global manifestations such as shallowing of rivers.
Of course, such a colossal process is influenced not only by the climate. Although, we must pay tribute, it really warmed up over the last hundred years. Due to the instability of winter temperatures, the snow cover becomes very thin by spring. As a result, the actual absence of floods - the main source of water resources for the riverbed.
Shoaling of rivers
In hydrology, we are talking about a whole range of problems that lead to a decrease in water resources. This problem is especially acute now in the basin of the main Russian river Volga. Every year it acquires new shoals. Water levels in reservoirs are falling catastrophically, transport volumes are decreasing every season.
There are many reasons for this. Not the last role is played by the human man-made factor. They stopped cleaning the river, as they regularly did it30 years ago. Trees and forest belts along the banks are being actively cut down due to mass housing development. All this is steadily reflected in the water level. The fairway is changing, and the banks are intensively overgrown with willow. In a word, it is a mistake to sin only on climate change. Although even in them the trace of too active human activity is clearly visible.
Geography and Hydrology
Geography lessons give us an initial understanding of the separate and important science of hydrology. Students study maps of the area with the names and locations of rivers, seas and oceans. Surprisingly, the images in the atlas take into account periods of drought and ice. They are transferred to the cards, taking into account all the changes.
Low water is a very inaccurate indicator. Often, droughts occur at the same time, but the water level in the channel can vary dramatically. These figures are more volatile than stable.
It is interesting that surveyors and geologists use data on the water level in rivers in their practice. Mainly to determine the level of groundwater occurrence and the number of sources that feed the channel. Once all the data is systematized, scientists can recreate a complete picture of climatic and natural resources: estimate the rate of water change in rivers and the period during which water from the source reaches the mouth, and even calculate the rate of the water cycle in nature. We can safely say that low water is not only a phase of the water regime, but also an important indicator for calculations in many areas of science.
