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Let's say your portal is 10km below sea level. Dropping from that pressure to pressure at sea level gives a flow speed of somewhat over 400 meters per second: $\sqrt{10^4m * 10\frac{m}{s^2} * 2}$ (water is incompressible so we can just use potential energy). This is well over the speed of sound, or comparable to the speed of a typical handgun bullet.
This 400 m/s flow is though the entire portal, $\pi * 5000^2 = 7.9 * 10^7 m^2$, for a total of about $30*10^9 m^3$ of water per second, that's a cube of water about 3 km or 2 miles on a side per second.

At this point most of my assumptions are starting to break down. I assumed the effect on the ocean surface would not be too great. It will likely be a giant maelstrom tens or maybe even hundreds of kilometers across. This means the amount of water flowing through it is going to be somewhat less. Let's cut it by a factor 10, so $30*10^8 \frac{m^3}{s}$.

The specific heat of water is about 4 J/(K kg), so it takes about 4 kilojoules to heat a Liter of water 1 degree Celsius. That's about 4 MJ to heat a cubic meter 1 degree. The potential energy in dropping a cubic meter of water from 10 km up is about 100 MJ, so you're going to heat up your water about 25 degrees by slamming it high speed into the sand (or, quite quickly, other water).

Water has a latent heat of about 2.3 MJ/kg, or 2300 MJ/m³. So it then takes a few centuries for all the water to boil off and turn into vapor.

Let's say your portal is 10km below sea level. Dropping from that pressure to pressure at sea level gives a flow speed of somewhat over 400 meters per second: $\sqrt{10^4\mathrm{m} \cdot 10 \frac{\mathrm{m}}{\mathrm{s}^2} \cdot 2}$ (water is incompressible so we can just use potential energy). This is well over the speed of sound, or comparable to the speed of a typical handgun bullet.
This 400 m/s flow is though the entire portal, $\pi \cdot 5000^2 = 7.9 \cdot 10^7 \mathrm{m}^2$, for a total of about $30 \cdot 10^9 \mathrm{m}^3$ of water per second, that's a cube of water about 3 km or 2 miles on a side per second.

At this point most of my assumptions are starting to break down. I assumed the effect on the ocean surface would not be too great. It will likely be a giant maelstrom tens or maybe even hundreds of kilometers across. This means the amount of water flowing through it is going to be somewhat less. Let's cut it by a factor 10, so $30\cdot10^8 \frac{\mathrm{m}^3}{\mathrm{s}}$.

The specific heat of water is about $4 \frac{\mathrm{J}}{\mathrm{kg \cdot K}}$, so it takes about 4 kilojoules to heat a Liter of water 1 degree Celsius. That's about 4 MJ to heat a cubic meter 1 degree. The potential energy in dropping a cubic meter of water from 10 km up is about 100 MJ, so you're going to heat up your water about 25 degrees by slamming it high speed into the sand (or, quite quickly, other water).

Water has a latent heat of about $2.3 \frac{\mathrm{MJ}}{\mathrm{kg}}$, or $2300 \frac{\mathrm{MJ}}{\mathrm{m}^3}$. So it then takes a few centuries for all the water to boil off and turn into vapor.

Let's say your portal is 10km below sea level. Dropping from that pressure to pressure at sea level gives a flow speed of somewhat over 400 meters per second: $\sqrt{10^4m * 10m/s^2 * 2}$ (water is incompressible so we can just use potential energy). This is well over the speed of sound, or comparable to the speed of a typical handgun bullet.
This 400 m/s flow is though the entire portal, $\pi * 5000^2 = 7.9 * 10^7 m^2$, for a total of about $30*10^9 m^3$ of water per second, that's a cube of water about 3 km or 2 miles on a side per second.

At this point most of my assumptions are starting to break down. I assumed the effect on the ocean surface would not be too great. It will likely be a giant maelstrom tens or maybe even hundreds of kilometers across. This means the amount of water flowing through it is going to be somewhat less. Let's cut it by a factor 10, so $30*10^8 m^3/s$.

Let's say your portal is 10km below sea level. Dropping from that pressure to pressure at sea level gives a flow speed of somewhat over 400 meters per second: $\sqrt{10^4m * 10\frac{m}{s^2} * 2}$ (water is incompressible so we can just use potential energy). This is well over the speed of sound, or comparable to the speed of a typical handgun bullet.
This 400 m/s flow is though the entire portal, $\pi * 5000^2 = 7.9 * 10^7 m^2$, for a total of about $30*10^9 m^3$ of water per second, that's a cube of water about 3 km or 2 miles on a side per second.

At this point most of my assumptions are starting to break down. I assumed the effect on the ocean surface would not be too great. It will likely be a giant maelstrom tens or maybe even hundreds of kilometers across. This means the amount of water flowing through it is going to be somewhat less. Let's cut it by a factor 10, so $30*10^8 \frac{m^3}{s}$.

Let's say your portal is 10km below sea level. Dropping from that pressure to pressure at sea level gives a flow speed of somewhat over 400 meters per second: sqrt(10^4m * 10m/s² * 2) (water is incompressible so we can just use potential energy). This is well over the speed of sound, or comparable to the speed of a typical handgun bullet.
This 400 m/s flow is though the entire portal, pi * 5000^2 = 7.9 * 10^7 m², for a total of about 30*10^9 m³ of water per second, that's a cube of water about 3 km or 2 miles on a side per second.

Comparing that to other rivers. The discharge of your portal is about 100 000 kubic kilometers per hour, or about three times the the amount of water discharged by all of Earths rivers in a year.

At this point most of my assumptions are starting to break down. I assumed the effect on the ocean surface would not be too great. It will likely be a giant maelstrom tens or maybe even hundreds of kilometers across. This means the amount of water flowing through it is going to be somewhat less. Let's cut it by a factor 10, so 30*10^8 m³/s.

Let's say your portal is 10km below sea level. Dropping from that pressure to pressure at sea level gives a flow speed of somewhat over 400 meters per second: $\sqrt{10^4m * 10m/s^2 * 2}$ (water is incompressible so we can just use potential energy). This is well over the speed of sound, or comparable to the speed of a typical handgun bullet.
This 400 m/s flow is though the entire portal, $\pi * 5000^2 = 7.9 * 10^7 m^2$, for a total of about $30*10^9 m^3$ of water per second, that's a cube of water about 3 km or 2 miles on a side per second.

Comparing that to other rivers. The discharge of your portal is about 100 000 cubic kilometers per hour, or about three times the the amount of water discharged by all of Earth's rivers in a year.

At this point most of my assumptions are starting to break down. I assumed the effect on the ocean surface would not be too great. It will likely be a giant maelstrom tens or maybe even hundreds of kilometers across. This means the amount of water flowing through it is going to be somewhat less. Let's cut it by a factor 10, so $30*10^8 m^3/s$.