The term "low water" comes from the concept of "boundary", that is, "border". However, it is directly related to hydrology. And if the flood indicates an excess of the normal water level, then the low water, on the contrary, indicates a decrease. What are the causes of these processes, what consequences can they have, and what role does man play in all this?
Term history
We have already mentioned the nature of the origin of the term. Indeed, the state of rivers, and indeed of nature as a whole, in ancient times was a certain boundary and the boundary of the transition from one season to another. Therefore, low water - it was a kind of calendar. Not without reason, after all, all the most ancient civilizations arose in river beds. 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 condition of the channel began to play a key role in shipping, because all the trade and passenger flows were carried through them for millennia. And if you think about it, the low water of the river this movement greatly complicated. It was from here that the famous barge haulers went. Whole gangs of peasant men pulled huge barges on the ropes, often also loaded with goods. And on the river Sura there were even female artisans of barge haulers.
Why does the low water occur
In hydrology, there are several explanations for the occurrence of a low water period. First of all, these are natural causes of a decrease in water availability, when the entire influx of water occurs solely from groundwater. That is, in such seasons when there is no flood. Traditionally, it is winter and summer. Precipitation in these seasons cannot provide the right amount of water, so the bed becomes shallow. Especially characteristic is the summer low water for the southern regions, where small rivers can even dry out, and vegetation even appears at the bottom of the channel.
The so-called vegetation period also affects the state of the channel and the occurrence of low water. This is a period of time when along the banks and at the bottom of rivers actively grow, and then all kinds of plants die. Too much vegetation affects the state 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 availability. The speed of the current, the depth of the stream and the type of bottom soil can also become determining.
The dependence of the water level on the time of year
But whatever one may say, seasonality for rivers plays a key role. Most sources agree that low water is mainly a seasonal phenomenon. The period of the lowest level of river water content averages at least 10 days.
In most temperate regions, low water starts in late summer and continues until freezing begins. An exception may be mountainous areas, where heavy rains often cause high water and a sharp increase in river water.
But unambiguously determining the terms of the low-water period can be difficult. Summer low water can stretch from the end of the spring flood to the very beginning of prolonged autumn rains. That is, actually for the whole season. And in the case of a small amount of precipitation, go to the autumn low water. The only season that is certainly not characterized by this phenomenon is spring, when melting snow completely compensates for any moisture deficit.
Winter low water
It is a completely different matter in the winter season, when hydrologists distinguish a special condition of the riverbed. On a significant part of the continent, stable ice formation remains for six months. The most critical marks of the cold season are November and December, that is, the period of ice cover formation. On average, a winter low-water season can last up to 170 days. Smaller waterways can freeze through, especially if they flow through karst areas.
During the winter low-water season, while the river is covered with ice, as well as in the summer-autumn period, in conditions of lack of precipitation, it feeds exclusively from groundwater sources.
Low water of rivers in different climatic zones
The climate of the watercourse is fundamentally affecting the nutrition of rivers, its type and character. As we have already found out, in a temperate climate they mainly have mixed food - 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, seasonality also takes place, and the duration and intensity of the rainy season is of greatest importance. Underground sources are practically absent here. Whereas in the tropical zone, rivers are filled almost exclusively due to underground nutrition.
A typical example of the temperate river is the Volga, which spreads in the spring and becomes shallow by the end of summer. The classical equatorial is the Amazon, which is actively replenished from December to April, when more than 60% of precipitation falls on this territory.
How is the low water determined?
We are entering a professional hydrological territory. For a common understanding, the most common criterion called “change in runoff volume” is enough. The time when this volume decreases to 15% of the annual volume can be considered a low water period. It is noteworthy that science considers the most stable is the decrease in the water level in the channel. In this regard, on large topographic maps, the rivers are depicted exactly as they are during the low water period.
If we talk about the flat streams of the middle strip, they are characterized by a low water level in the summer. And, for example, in the mountains, spring snowmelt stretches for the whole season and captures the summer, when the ice cover begins to melt. Therefore, at mountain rivers there are practically no low low-water levels. They always retain a sufficient supply of moisture, including due to heavy and frequent rainfall. In the Far Eastern region, where floods are frequent even in hot weather, summer low water levels are also rarely observed. This is not surprising, because rainfall there are frequent and heavy.
Global processes
Summer and winter low water are not only constantly repeating processes, like changing seasons. Climatic factors, such as long droughts in the summer and lack of rainfall in the winter, can cause more global manifestations, such as shallowing of rivers.
Of course, such a colossal process is affected not only by the climate. Although we must pay tribute, it really has warmed over the past hundred years. Due to the instability of winter temperatures, the snow cover becomes very thin by spring. As a result, the actual absence of flood is the main source of water resources for the river bed.
River shallowing
In hydrology, we are talking about a whole range of problems that lead to a decrease in water resources. This problem is especially acute in the basin of the main Russian river Volga. Every year it acquires new shoals. The water level in the reservoirs is falling dramatically, the volume of transport is reduced 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 30 years ago. Trees and forest belts are actively cut down along the banks due to massive housing development. All this is steadily reflected in the water level. The fairway is changing, and the banks are intensely overgrown with talnik. In a word, to sin only on climate change is a mistake. Although even in them a trace of too active human activity is clearly visible.
Geography and hydrology
The lessons of geography give us an initial idea of the separate and important science of hydrology. Pupils study maps with the names and locations of rivers, seas and oceans. Surprisingly, the images in the atlas take into account periods of drought and freezing. They are transferred to the cards taking into account all 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 indicators are more volatile than stable.
Interestingly, data on the water level in rivers are used in their practice by surveyors and geologists. Mainly to determine the level of groundwater and the number of sources that feed the channel. Once all the data is systematized, scientists can recreate the full picture of climatic and natural resources: estimate the rate of change of water in rivers and the period for which water from the source reaches the mouth and even calculate the speed of the water cycle in nature. We can safely say that the low-water period is not only a phase of the water regime, but also an important indicator for calculations in many fields of science.