Monsoons Shaping the Desert Landscape

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Although Monsoon storms are intermittent, the impact on the desert landscape is lasting. Arizona State University geomorphologist Kelin Whipple will tell us more about how sudden summer storms can even move boulders.

TED SIMONS: AND THE PHOENIX AREA WAS SLAMMED BY MONSOON RAIN THIS MORNING. STREET FLOODING AND OVERFLOWING WASHES WERE REPORTED IN SEVERAL CITIES INCLUDING MESA, AVONDALE, SOUTH PHOENIX, BUCKEYE, AND GOODYEAR, WITH APACHE JUNCTION APPEARING TO GET THE WORST OF IT AND AHWATUKEE GETTING CLOSE TO TWO INCHES OF RAIN WITHIN AN HOUR. THERE WERE ALSO AT LEAST A COUPLE OF REPORTED SWIFT WATER RESCUES.MONSOON-TRIGGERED RAINS ARE INTERMITTENT AROUND THE STATE, BUT THE IMPACT ON THE DESERT LANDSCAPE IS LASTING. TO TELL US HOW THESE STRONG SUMMER STORMS CAN LITERALLY SHAPE THE DESERT IS ASU GEOMORPHOLOGIST KELIN WHIPPLE. WELCOME TO ARIZONA HORIZON, GREAT TO HAVE YOU HERE.

KELIN WHIPPLE: THANK YOU TED, IT'S GREAT TO BE HERE.

TED SIMONS: BEFORE WE GET STARTED, WHAT IS A GEOMORPHOLOGIST?

KELIN WHIPPLE: A GEOMORPHOLOGIST STUDIES THE EVOLUTION OF THE LANDSCAPE, SO GEOMORPHOLOGY IS THE SHAPE OF THE EARTH, SO WE STUDY LANDSCAPES AND HOW THEY ARE FORMED LONG AND SHORT TERM.

TED SIMONS: AS FAR AS A DESERT IS CONCERNED AND WATER, WHICH YOU DON'T NORMALLY ASSOCIATE WITH A DESERT, A LOT OF WHAT YOU SEE HAS BEEN SHAPED BY WATER.

TED SIMONS: YES, THAT'S CORRECT. ALL LANDSCAPES, WATER IS THE DOMINANT PROCESS OF SHAPING THE LAND FORMS.

TED SIMONS: AS FAR AS THE MONSOON IS CONCERNED, THAT'S WHEN THE BIG TIME SHAPING OCCURS, CORRECT?

KELIN WHIPPLE: IT'S NOT THAT CLEAR. ONE OF THE THINGS ABOUT MONSOON STORMS IS THEY ARE INTENSE BUT BRIEF, AND MONSOON SYSTEMS MOVE THROUGH QUICKLY. MOST PLACES ONLY GET RAINED ON FOR A SHORT TIME. YOU HAVE INTENSE PEAK FLOWS BUT THEY OFTEN ONLY LAST FOR A FEW HOURS. SO WHEN YOU GO TO SHAPE THE LANDSCAPE YOU WANT TO LOOK AT BASICALLY THE TOTAL NUMBER OF HOURS OVER A THOUSAND YEAR PERIOD THAT FLOWS HAD A HIGH ENOUGH VELOCITY TO MOVE BOULDERS AND ERODE THE BED. THE LIMITING THING FOR MONSOONS IS HOW BRIEF THEY ARE. THEY'RE REALLY INTENSE BUT IT'S NOT ENTIRELY CLEAR THEY DO THE MOST WORK, BUT THEY ARE AN IMPORTANT PLAYER.

TED SIMONS: WE HAVE A PHOTO OF A STREAM. I WOULD IMAGINE IF YOU ARE SAYING SOMETHING HAS TO BE THERE FOR A LONG PERIOD OF TIME AND SUSTAINED, DOES THIS KIND OF THING MAKE A DIFFERENCE?

KELIN WHIPPLE: ABSOLUTELY. THIS IS A CANYON OUT NEAR BAGDAD, ARIZONA YOU ARE LOOKING AT. AND THOSE CANYONS WERE CUT JUST DURING THE LAST MILLION YEARS. THAT'S A SHORT TIME GEOLOGICALLY, HARD TO IMAGINE FOR MOST PEOPLE, BUT AS IT'S CUT DOWN, IT'S ARMORED IN THOSE VERY LARGE BOULDERS. THEY ARE A METER IN DIAMETER. FOR THAT RIVER TO CONTINUE TO ERODE DEEPER, YOU HAVE TO HAVE FLOWS WITH ENOUGH VELOCITY TO GET THE BOULDERS MOVING SO THE WATER CAN ATTACK THE BED.

TED SIMONS: ONE TIME THOSE ROCKS AND BOULDERS WERE SOMEWHERE ELSE, WEREN'T THEY?

KELIN WHIPPLE: CORRECT. THEY WERE UP ON THE SIDES OF THE HILLSIDE AS IT ERODED DOWN THEY'VE TUMBLED IN AS LANDSLIDES AND DEBRIS FLOWS AND NOW THEY ARE GETTING ROUNDED AND SLOWLY MOVING DOWN THE RIVER.

TED SIMONS: WE HAVE ANOTHER PHOTO, THIS IS OF A DRY WASH. LOOK AT THIS THING. THIS THING WAS MOVED BY WATER? WHAT IS THAT?

KELIN WHIPPLE: THAT'S A FOOTING OF A PIER THAT YOU CAN SEE STICKING OUT, THAT BIG METAL ROD, IT'S A SUPPORT BEAM FOR A PIPE THAT YOU'LL SEE IN ANOTHER PHOTO. SORT OF LIKE A BRIDGE FOOTING. AND THAT WAS RIPPED OUT EVEN THOUGH IT'S A COUPLE OF METERS IN ONE DIMENSION, FOUR METERS IN THE OTHER DIMENSION. AND MOVED DOWN IN A FLOOD WE BELIEVE IN THE LATE '70S.

TED SIMONS: THAT RECENT?

KELIN WHIPPLE: THAT RECENT. NOTHING MUCH ELSE HAPPENED IN THAT TIME.

TED SIMONS: SO IF SOMETHING THAT BIG CAN MOVE THAT MUCH AND MOVE AT ALL, THAT MEANS THAT, THAT WATER, OVER TIME, YOU ARE TALKING MILLIONS OF YEARS HERE, MAKES A BIG DIFFERENCE.

KELIN WHIPPLE: ABSOLUTELY. YOU SAW THE ENTIRE CANYON'S BEEN CUT IN THE LAST FEW MILLION YEARS.

TED SIMONS: ANOTHER SHOT HERE OF A DRY WASH, I THINK THAT'S THE SAME GENERAL AREA, SAME GENERAL WASH, AND THERE'S YOUR FOOTING. IT'S NOT LAYING ON THE GROUND.

KELIN WHIPPLE: YEAH, EXACTLY. SO THIS IS A SHOT TAKEN FROM THE POSITION OF THAT LAST FOOTING THAT'S MOVED SHOWING YOU WHERE IT MOVED FROM. SO IT'S COME 50-METERS DOWNSTREAM. MOVED IN ONE EPISODE IN THAT BIG FLOOD. A LOCAL RANCHER TOLD US IT WAS A BIG FLOOD THAT HE REMEMBERS AS A CHILD IN THE LATE ‘70S.

TED SIMONS: I THINK ANYONE HERE IN THE LATE '70S REMEMBERS WE HAD A TON OF FLOODING. ALL THE BRIDGES AROUND THE SALT RIVER EXCEPT FOR THE MILL AVENUE ONE WERE SHUT DOWN. THAT MIGHT HAVE BEEN THE SAME TIME?

KELIN WHIPPLE: IT COULD WELL BE.

TED SIMONS: THIS PHOTOGRAPH, YOU LOOK AT THESE ROCKS AND BOULDERS AND REALIZE THEY WEREN'T ALWAYS THERE. THE DESERT IS BEING SHAPED BY THIS MASSIVE AMOUNT OF RUNNING WATER. THIS IS A WIDER SHOT OF THE CANYON. TELL US WHAT WE ARE LOOKING AT. WHAT ARE WE LOOKING AT HERE?

KELIN WHIPPLE: THIS IS THE SAME -- WE WERE DOWN INSIDE THE CANYON IN THE LAST PHOTO. THE CANYON IS ABOUT 600 METERS DEEP. IT WAS ALL CARVED WITHIN THE LAST 8 MILLION YEARS. WE KNOW THAT BECAUSE THE TOP OF THE CANYON IS AN 8-MILLION-YEAR-OLD LAVA FLOW THAT POURED OUT AND FILLED THE AREA TO A FLAT LEVEL, AND THE ENTIRE CANYON HAS BEEN CUT IN THAT TIME.

TED SIMONS: SO BASICALLY SOMETHING, SOMETIME BACK IN THE PAST, THERE WAS WATER-SO WHAT, AT THE LEVEL WE CAN SEE THAT CANYON CUT, AT THE UPPER LEVELS?

KELIN WHIPPLE: AT THE UPPER LEVELS, THERE WOULD HAVE BEEN JUST A LITTLE SHALLOW STREAM RUNNING ALONG THE TOP THERE. AS YOU COME DOWNSTREAM, IT PROBABLY HAD A STEEPER SECTION AS IT DROPPED INTO THE VALLEY. AND OVER TIME THAT STEEP SECTION HAS CUT ITS WAY UPSTREAM LIKE IT'S UNZIPPING. WE BELIEVE THOSE SORT OF CANYONS CUT VERY RAPIDLY INITIALLY, THEY ACCUMULATE THAT BOULDER ARMOR ON THE BOTTOM, AND THEN IT SLOWS DOWN. AT THAT POINT THE RIVER HAS A GENTLER SLOPE AND IT'S GOT ALL THESE BIG BOULDERS TO DEAL WITH. NOW YOU ARE WAITING FOR A 20 YEAR RETURN AND FLOOD TO JUST MOVE SOMEE BOULDERS.

TED SIMONS: SO THESE MASSIVE FLOODS, THE BIG ONES, THE ONES THAT MAKE A DIFFERENCE IN TERMS OF SHAPING THE LANDSCAPE, HOW FREQUENT?

TED SIMONS: THAT'S A GREAT QUESTION. IT'S SOMETHING THAT WE'RE STILL WORKING TO UNDERSTAND AS WELL AS WE CAN. IT NEEDS TO BE A FLOOD THAT WILL GENERATE VELOCITY BEYOND THE CRITICAL VALUE TO GET THE COBBLES OR BOULDERS MOVING. AND TYPICALLY THOSE FLOWS RETURN MAYBE OVER TWO YEARS AT A MINIMUM, TWO YEARS OR GREATER. AND SO WE THINK USUALLY, OVER THE LONG TERM YOU HAVE A TRADE-OFF LIKE I TALK ABOUT IN GEOMORPHOLOGY BETWEEN THE MAGNITUDE OF EVENTS AND THE FREQUENCY OF THOSE EVENTS. IF YOU CONSIDER A 10 THOUAND YEAR PERIOD, YOU WANT TO ADD UP THE TOTAL NUMBER OF DAYS WHERE YOU WERE DOING SOMETHING.

TED SIMONS: OKAY. WITH THAT IN MIND, IS THERE A CUMULATIVE EFFECT REGARDING THE STORMS WE'VE HAD IN THE PAST COUPLE OF DAYS? WE'VE HAD MONSOON STORMS. MAYBE NOT MASSIVE CHANGE, BUT CHANGES NONETHELESS.

KELIN WHIPPLE: ABSOLUTELY. MONSOON STORMS ARE NOT ACTUALLY THE BIGGEST STORMS WE TYPICALLY GET IN THE VALLEY, AT LEAST WITH DAILY PERIODS, BUT FOR SHORT DURATION PERIODS, THEY ARE SOME OF THE BIGGEST FLOODS. SO THEY WILL MOVE MATERIALS, THEY WILL DEPOSIT SEDIMENTS, THEY'LL CAUSE RIVERS TO EVULSE WHICH MEANS THE RIVER GOES FROM A PREVIOUS PATH TO A NEW PATH. THAT'S OFTEN WHERE YOU SEE A SERIOUS FLOODING HAZARD, WHERE A CHANNEL JUMPS TO A NEW PLACE YOU HADN'T EXPECTED IT TO BE.

TED SIMONS: ISN'T THAT INTERESTING? LAST QUESTION HERE. MASSIVE FLOODS. THE BOULDER MOVERS, THOSE THAT CLIMB UP HIGH, THOSE SORT OF THINGS. THE PHOENIX AND TUCSON AREAS, ARE WE AT RISK FOR A MEGA FLOOD?

THE BEST EXAMPLE OF THE KIND OF FLOODS PHOENIX CAN EXPERIENCE ARE THE ONES ON THE SALT RIVER. THE LAST ONE WAS IN THE 90S AT SOME POINT. WE KEEP THE SALT RIVER CONTROLLED BY THE VARIOUS DAMS ON IT. IF YOU GET ENOUGH RAIN AND THE DAMS GET HIGH ENOUGH, YOU CAN'T RISK A DAM OVER TOPPING CAUSE THEN YOU CAN LOSE THE DAM. YOU JUST HAVE TO LET THE WATER GO. SO WE CAN AND WILL AGAIN HAVE BIG FLOODS ON THE SALT RIVER, NO QUESTION ABOUT THAT.

TED SIMONS: THE LANDSCAPE CHANGES DOWN RIVER AND WE LIVE WITH IT. INTERESTING STUFF. GOOD TO HAVE YOU HERE. THANKS FOR JOINING US.

KELIN WHIPPLE: THANK YOU FOR THE OPPORTUNITY. I REALLY APPRECIATE IT.

TED SIMONS: UP NEXT ON "ARIZONA HORIZON," WE HEAR ABOUT THE FATHER OF PHOENIX FREEWAYS.

Kelin Whipple: Geomorpholigist

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