How are salt lakes formed? A friend and I are having a heated
discussion. He thinks that the salts were already in the basin, a
remainant of a sea previously there. I think the salts are brought in
by the river or rivers draining into the basin carrying the minerals
and leaving them behind as the water evaporates.

Hello.

You are correct.

An explanation from the University of Texas at Dallas essentially
mirrors your views:

"The Great Salt Lake, Dead Sea, and other salt lakes have no outlets.
All the water that flows into these lakes escapes only by evaporation.
When water evaporates, the dissolved salts are left behind. So a few
lakes are salty because rivers carried salts to the lakes, the water
in the lakes evaporated and the salts were left behind. After years
and years of river inflow and evaporation, the salt content of the
lake water built up to the present levels."
http://www.utdallas.edu/~pujana/oceans/why.html

Here is an additional description of the Dead Sea from
extremescience.com:

"The Dead Sea is continually fed water from the rivers and streams
coming down off the mountains that surround it. But the kicker is
this....no rivers drain out of the Dead Sea. The only way water gets
out of the Sea is through evaporation. And boy does it evaporate! This
part of the world get plenty hot. When the water evaporates, it leaves
behind all the dissolved minerals in the Sea, just making it saltier.
In fact, it's through the dual action of; 1) continuing evaporation
and 2)minerals salts carried into the Sea from the local rivers, that
makes the Sea so salty. The fact that the water doesn't escape the Sea
just traps the salts within its shores."
http://www.extremescience.com/DeadSea.htm


At the same time, if it's any consolation to your friend, he's not
completely wrong. In some cases, a salt lake is a remnant of a
previously much larger lake on the same site. The Great Salt Lake in
Utah is an example.

Here is an explanation from Earthsky.com:

"Why is the Great Salt Lake so salty? It's the remnant of a great
prehistoric body of water called Lake Bonneville. As the climate
changed, getting drier over the past tens of thousands of years, the
ancient lake shrunk. Because there is no outlet leading to the sea,
much of the dissolved salts had nowhere to go; much of it was laid
down as dry salt deposits while the rest became more and more
concentrated in a smaller volume of water. Even today, salts continue
to enter the lake from rivers that flow into it."
http://www.earthsky.com/Features/Articles/brine-shrimp1.html

The University of Colorado geography department has a fascinating
graphic of what Lake Bonneville might have looked like at its greatest
extent. It covered nearly half the state of Utah!
http://www.colorado.edu/geography/virtdept/module/GSL/lakelevels1.gif

Of course, the salt content of the lake not only is due to it being a
concentrated remnant of a larger and older lake, but it's also the
result of the continuing deposit of salts by river water flowing into
the basin.

Here is a further explanation from the US Geological Survey:

"The Great Salt Lake is the remnant of ancient Lake Bonneville , which
gave the Bonneville Salt Flats their name. Geologists estimate that
Lake Bonneville existed between 23,000 and 12,000 years ago, during
the last glacial period. Lake Bonneville's existence ended abruptly
when the waters of the lake began to drain rapidly through Red Rock
Pass in southern Idaho into the Snake River system. As the Earth's
climate warmed and became drier, the remaining water in Lake
Bonneville evaporated, leaving the highly saline waters of the Great
Salt Lake. The reason for the high concentration of dissolved minerals
in the Great Salt Lake is due to the fact that it is a "terminal
basin" lake; water than enters the lake from streams and rivers can
only leave by evaporation. As the process occurs over time, the
dissolved substances in the river water become increasingly
concentrated."
http://vulcan.wr.usgs.gov/Glossary/Glaciers/IceSheets/description_lake_bonneville.html

search strategy: great salt lake, dead sea, evaporation, formed,
concentrated salt, salt content, lake bonneville

I hope this helps.

Living in Utah, along the Salt Lake, I know a few things about this
topic. Lake Bonneville was actually a relatively freshwater lake, with
salt on the bottom, salt keeps comming into the current lake also from
underneath.

Their are actually two parts of the Great Salt Lake, the Northern Arm,
and Southern Arm, devided by a railroad causeway.  The Northern Arm is
twice as salt as the southern arm due to the lack of freshwater being
dumped into it.  Beleive me, I have swam in both, you haven't been
anywhere more salty than the northern arm.

If you want proof, go here:
http://www.ugs.state.ut.us/online/PI-39/

For a great picture of the Great Salt Lake, go here:
http://visibleearth.nasa.gov/cgi-bin/viewrecord?11816

More:
http://visibleearth.nasa.gov/U_S__States/Utah/

An rather unusual example of salty lakes is the Salton Sea in
California's Imperial Valley.  The Salton Basin was a typical dry,
desert lake bed until an irrigation project failure rerouted the
Colorado river's entire flow into it, forming the Salton Sea.  In the
beginning, the body of water was fresh, but salt deposits beneath the
Sea, which were mined before the flooding, disolved until after almost
a century, the lake is decidedly saline.  Please see the Salton Sea
Authority's web page at
     http://www.saltonsea.ca.gov/histchron.htm

Creeks and rivers typically only have a few parts per billion of salt,
but occasionally the creek flows though a salt bed, and parts per
thousand occur. The salt bed is likely part of a fossil salt lake, so
the salt is the result of a salt lake somewhere and sometime ago. This
is likely the usual source of the salt, but on rare occasions, the
salt comes mostly from the lake bottom.  Neil