Water and Lake
Water and Lake are two of the Eight Trigrams, The Eight trigrams represent all possible natural phenomena in the universe. They are the product of the interaction of Qian Heaven and Kun Earth. Kun and Qian have six children and together that makes eight.
In Chinese Medicine the most important of these Trigrams from the I Ching are, Qian Heaven, Kun Earth, Kan Water, Li Fire, and Xun Wind. Yet, it is Dui Lake that gives us a better understanding of our physical, mental and spiritual potential. Dui Lake also represents our connection to nature and the way it manifests, which is called Dao in Chinese.
To start the discussion I will use the same question and answer format found in the Yellow Emperor’s Classic. Would Huang Di approve?
Huang Di asked, What is the difference between Diu lake and Kan water?
Qi Bo knocked his head to the ground three times out of reverence.
Qi Bo answered, A wise question indeed. I shall answer it to the best of my abilities.
In Kan water the yang is stored.
In Dui lake the yang is potential.
Water

Trigram 6 is Water (kan or k'an), is one of the Five Chinese Elements. The trigram represents the abysmal, or the gorge, since that's where the water naturally travels. The nature of water is to flow to the lowest place, as is pointed out in Chinese philosophy — for example the Dao De Jing.
Yet, it is Yang that creates living Water with its transforming nature. Which is represented by its Yang middle line.
Lake

Trigram 2 is the Lake (duì or tui), the big body of water. The trigram represents the joyous and open.
The Earthly Yin resting on two Heavenly Yang. Yet, the Lake is an image of the sea, the primordial sea that most ancient creation myths contain as a primordial element. Since Heaven is the creative force in Chinese tradition, it must lie beneath what seems to be the origin of everything. So, beneath the depth of the ocean hides the creative first cause: Heaven.

Just like Laozi when he leaves China by way of a pass to the West. The West represents the return to the source. Dui lake represents the source. Which is why Dui lake is placed in the West in the Later-Heaven arrangement of the Eight trigrams.

The source is the eternal Dao. The unity with non-being.
Kan Water and Dui Lake share the same location, the West. Kan Water is placed in the West in the Early-Heaven arrangement of the Eight Trigrams.
The hexagram Chien 60 is Water on top of Lake. It represents limitations. Because to know ones limitation brings contentment. Lake is potential and the Yin Earth line in the trigram of Lake is the limitation.
A Lake occupies a limited space. When more Water comes into it, it overflows. Therefore limits must be set for the Water. The image shows Water below and Water above, with the firmament, a Yin line, between them as a limit.

Children
The Classic of Changes, Qian heaven trigram and the Kun earth trigram gave birth to six offspring.
The three male offspring, Zhen Thunder, Kan Water, Gen Mountain.
The three female offspring, Xun Wind, Li Fire, Dui Lake.
Dui Lake is a female child of Qian Heaven and Kun Earth. This may seem contradictory because the hexagram for Dui Lake contains two Yang lines on the bottom and one Yin line on top. So it seems that Dui Lake is more Yang then Yin. But, it is potential Yang.
In the Dao De Jing Water is being compared to number one and called the Great. It is called number one because it is the first or primordial. It is the potential for all life. Without the source of Water there would be no life.
This is reflected in the statement from the Dao De Jing.
I do not know its proper name but will call it Dao. If forced to give it a name I shall call it great. Great means moving away, moving away means far away, and far away means ultimately to return.
If forced to give the Dao a name we must call it Great. Great means the unlimited intelligence of the universe that creates all things, but does not lord over them.
The basic numbering of the universe is described in Chapter 42 of the Dao De Jing in four stages. It is moving from the unity of non-being Dao to being the myriad things.
The fifth stage is the axis around which the Dao’s movement occurs. The movements of the Dao are the cosmological basis for the Five Agents of Transformation, or Five Elements.
1 is Water which is Great
2 is Fire which is moving away
3 is Wood which is far away
4 is Metal which is the return
5 is Earth which is the axis
The first stage of Dao corresponds to Water which represents the potential basis for all manifestations in life. Without Water there would be no life, only dryness.
The generation of Fire from Water corresponds to moving away from the original nature.
These two poles of Fire and Water generate a third stage which corresponds to Wood. The function of Wood is to blend Water and Fire back into one by harmonizing.
The fourth stage signifies a return from the duality of Fire and Water back into the Original nature of Dao as unity. Every day and every year the Dao moves away, and returns to itself.
Creation of the Trigrams
If Qian Heaven has intercourse with Kun Earth, the Kan Water trigram is born.
The central line of Qian Heaven is given to Kun Earth and the trigram Kan Water emerges, and although Kun has changed to Kan, her body still exists.
Heaven mixing with Earth; the yang in the middle of Heaven ☰ enters the palace of Earth ☷, so that the trigram of Earth is filled in and becomes Water ☵.
Kun Earth and Kan Water dwell together, as Water and Earth are united. The virtue or effectiveness of Kun Earth is its storing ability, and this is also the virtue of Kan Water.
What do Earth and Water store, well Yang naturally. The Yang in Kan Water comes from Heaven, it is the True yang. It is also called, the Pre-natal Yang, Life Gate Fire, Dragon Fire, Ming men, Original fire, or the Life or Destiny fire.
Yang Fire should be drawn down and stored, and it is not suitable for it to fly about. Both Kun and Kan can store the Yang fire. When they fail to store Yang, there is disease.
The union of Water and Earth is not only important to the understanding of Tai yin, but also Shao yin. Tai yin is Earth and Water. Shao yin is Fire in Water.
As long as the True yang exists, there is life, and we are healthy. It the True yang is not stored away, or if the True yang is used to perform some other function, then the life-giving Qi is not warmed and nourished.
If it is not contained, and True yang flies upward and outward, then external syndromes indicating dangerous crises will rise accordingly.
Qi Bo looked at Huang Di to see if he understood. He did, Qi Bo was content.
Conclusion
Water descends and stores.
Lake opens and reflects.
Kan Water holds the Fire of life deep within the Earth, guarding the True Yang like an ember protected from the wind. Dui Lake, wide and luminous, gathers heaven’s light at the surface, revealing what may yet be born. One conceals, the other invites; one preserves life, the other gives it direction.
Together they teach us the art of balance. Without storage there is exhaustion; without limitation there is overflow. To know where to contain and where to open is to understand both medicine and Dao. In the body this is the safeguarding of life; in the spirit it is contentment; in nature it is the rhythm of departure and return.
Water moves away from the source, Lake returns us to it. Every cycle of health, every breath of living Qi, follows this same arc. When the Yang is rooted, life endures. When potential is honored, joy arises. Thus the Dao flows on, descending, rising, departing, returning - quietly completing itself in us, just as it does in Heaven and Earth.