Canals

Cylinder seal depicting he sun god Shamash,  Akkad (Iraq), 3rd millennium BCE, Chert. Currently in the collection of the Israel Antiquities Authority...

Official description:  Shamash emerging through a pair of mountains flanked by columns, perhaps heaven’s door...🙂 Maybe, but I doubt it. Here is why:

Modern impression from a greenstone cylinder seal from Sippar, c. 2300 BC...Currently in The British Museum... 

Here is Shamash (the sun god), in full power (represented by sun heat rays emanating from both shoulders), standing "between two columns with a lioness and a lion". "Between the lions" means that he is standing in Leo, end of July, beginning of August...

And, Leo is a solar year calendar marker which marks the beginning of the mating season of the Eurasian lions. 

Hence lioness and lion...

Leo is the hottest and driest part of the year in Mesopotamia. The time of maximum heat, maximum drought. 

This is also the time of the lowest water levels in Tigris and Euphrates river system...Water level charts for

Tigris

Euphrates

Which is why Shamash is standing in an empty river (canal) bed...And not between two mountains. 

What about the gates of heaven?  Well to understand what these "columns" really are, we need to know a bit about the Mesopotamian agriculture year and their irrigation practices. 

Have a look at the first seal. The image is to be read right to left. The first thing we see is a sheaf of grain. The harvest in ancient Mesopotamia started in April with barley cutting and ended in July with barley storing...

Right after that starts the hottest and driest part of the year dominated by Shamash, who is depicted standing in an empty river (canal) bed. In the "Traditional Dam Construction in Modern Iraq: A Possible Analogy for Ancient Mesopotamian Irrigation Practices" we can read that

Here is what this canal system looks like

Here is what the head dam looks like

And here is how the head dams are made:

The head-dam is built at the final stage of the construction of the primary canal, which is dug from tail to head toward the water source. The head-dam may reach a height of three to five metres, depending on the size of the primary canal and the difference in elevation between the canal bed and the river. 

The construction of a head-dam usually takes place between July and September, during the agricultural off-season and when water levels in the rivers are generally low. This allows for the construction of the dam to take place on relatively dry ground. 

Prior to construction, any loose silt and/or organic material is removed from the canal bed in order to provide a firm foundation for the dam. The foundation for the mud and reed part of the head-dam is built from either baked or unbaked bricks (compressed mud) or stone. On top of the foundation, a number of terracotta pipes are put in place to serve as the water inlet (A). The amount of water entering the primary canals can be regulated by closing the pipes with clay on either side. 

Several rows of palm-trunks or fruit-tree trunks are placed on top of the terracotta pipes in a criss-cross fashion (B). Depending on the length of the head-dam, a number of palm-trunks are sunk vertically into deep holes. More tree-trunks are then placed horizontally against the row of standing logs. The layer of horizontally placed tree-trunks may be up to three rows high. 

Alternating layers of mud (C), reed bundles13 and reed mats (D) and occasionally stones are placed on top of the rows of tree-trunks. The reed bundles, thinner at one end, are woven together to increase the head-dam’s stability.

So what are reed bundles? Only the most important building material in Mesopotamia. Used for building everything, from houses to irrigation canal dams...

Now look again at "the gates of heaven" from the above seals...Vertical bundles of reed tied together with horizontal reed ropes...And how do you cut reeds? With a serrated knife, which Shamash holds in his hand...By the way, the knife still used for cutting reeds is still serrated and curved...

Here is another seal depicting the sun god Shamahs. Cylinder seal, Akkad, 2340 B.C. and 2150 B.C. God carrying mace -- Sun god with rays, ascending between two mountains -- At either side, attendant opening wing of gate the gate of heaven...Currently in the collection of the Morgan Library and Museum...

More gates of heaven 🙂...Let's ignore this...This seal adds another interesting detail to the scene. Shamash is facing (walking towards) another god holding a mace...Well that can only be Hadad (Ishkur), the storm god...The rains arrive to Mesopotamia at the end of the hot dry half of the year, in Oct/Nov...

So the sun god Shamash, standing in the middle (Jul/Aug) of the hot, dry part of the year (Apr/May-Oct/Nov), is facing (walking towards) the Storm God Hadad, who comes at the beginning (Oct/Nov) of the cool, wet part of the year (Oct/Nov-Apr/May)...

Finally, here is something I discovered only the other day. I was looking at the reed boat building technology in Mesopotamia. I first read this paper "Building the reed-boat prototype: problems, solutions, and implications for the organization and structure of third-millennium shipbuilding" which documents building of a replica Bronze Age Mesopotamian reed boat. You can see that the main building block is again a reed bundle.

This lead me to this web page "Connecting Mesopotamia with Indus Valley and Egypt: The Tigris Expedition", which describes another experiment in reed boat building. While reading this second text, I came across this chapter:

In Iraq, Heyerdahl gathered evidence for the ancient use of maritime vessels made from a tall freshwater reed called "berdi".  There was more information, as well, on coatings the ancients may have used to protect these reeds against water absorption.  Asphalt in some kind of mixture with pitch and oil was mentioned.  

Much more importantly, Heyerdahl learned from the marsh Arabs of Iraq a vital piece of data on the performance of "berdi" that would influence the entire outcome of his planned experiment. Berdi, they said, must be cut in August, and only in August, or it absorbs water quickly and sinks.  The berdi cut in August was dried for two or three weeks and then used for the reed houses in which the Arabs dwelled.  Estimations of the buoyancy of properly harvested berdi ranged to upwards of a year.  This was a new, seasonal aspect of reed boat construction that no one had considered before.

Cut your reed for construction in August, in Leo...This is what Shamash is telling his people on the above seals...

No heavens gates...Just climate, biology, agriculture and technology...

PS: Part of a model chariot, with an impression of the sun god Shamash, not "rising over the mountains" as the official description says, but standing on top of the main canal dam (see the hole), telling people that it's time to go cutting reeds (see the knife he is holding) to repair the dams...

Old Babylonian, ca. 2000–1600 BC. currently in the Met Museum