Steve,
One word.
Bamboo.
Find a bamboo that will live in your conditions, probably a running or a Fargesia, (clumping), and plant at least a couple of them.
I have seen this work with my own eyes on a highway steep slope close to where I live.
I was amazed.
You can also cut it down in the spring if you would like, just not too low, and it will be fine, it will regrow.
Research online bamboo good for erosion, (it will hold that dirt in, trust me) their roots are as strong as steel.
Besides, it's pretty.
I like it very much, very tropical looking.
If the bamboo totally turns you off, you could always terrace the whole back yard.
VERY expensive, but that will work.
GOod luck, let us know what you choose to do.

Read link below, it's a good one.

Here is a link that might be useful: Erosion control on steep hillside

We have a steeply sloped backyard too. It is south/southwest facing, gets a lot of hot sun during the summer. Most of it is planted in junipers except for an empty patch that is really an eye sore. We had more junipers planted in that spot because we just can't get down there to trim or weed unless we tied ourselves up to a post and propelled down.

There is a zigzag pathway down our hill but the owner/builder really should have added an extra 'zag', because it's really steep. My dream would be to tear everything out, put in retaining walls, a seating area at each turn, a fountain or two too, and for my husband...a zip line all the way down! He's crazy.

Because at the bottom of the hill a creek runs along our property and it's just breathtaking! In fact, before we even walked in the house we took one look at the back yard, as steeply sloped as it was, and we knew we couldn't pass this property up.

Our slope looking up.

We've cleared the area by the birch tree and along the creek in the first pic, but on the hill there is still planting to do. Not only that, but the old rail ties that were used are rotting. It may not look like it, but there is a lot of work to do yet. These pics were taken in spring before we moved in, so the hillside looks pretty good only because the weeds in the middle of the hillside are green!

I don't have any ideas on the sliding of soil but goodbyekitty, I have a suggestion for you. If it was my property I would get couple of bags of post hole cement & some 1 or 1 1/2 in. metal pipe & make a railing so you have something to hang on to, if those railroad ties get mossy or wet could send you tumbling. Then if you could make the steps each longer, a little at a time,wouldn't seem so steep. A level spot after the bend with a comfortable chair so you could rest a bit would be nice. Long way up if you are hurt or 1 of your kids, always wise to have a comfortable place to stop, even a bench would be helpful, could match your deck. Better if it had a little shade Colorful shrub perhaps. GM had a back yard very much like that. Even as kids tho we would lay out a towel & rest half way up after swimming(lake) otherwise we got winded but at least it was grass & rocks. It wasn't mowed but summers were short. Bench would have really been nice! Lovely yard & creek is very nice, good fishing??

Thanks Sunnyca. We do get salmon or trout in the spring, but they move on or else the crane, ducks, geese, owls and muskrat get them. It's amazing to listen to a barn owl's hoot reverberate up and down the creek.

We need to do something to make it easier to get up and down that steep slope. Anything we do would be very labor intensive. My husband is afraid of having anything more than a deck party here because he thinks someone might get hurt walking up and down that hill. I'd like to put a couple of adriondack(sp) chairs and a table down there with a small grill. That would be fun. I just noticed something in the second photo; this was taken before we moved in but now the creek seems to have moved closer. There is a tree growing in the first photo that is on dry ground, but now the same tree is sitting in water. You can barely see it on the right bottom edge of the photo, it's just a sapling.

I once put my photo up for a show on HGTV called "Extreme Landscapes" or something like that? I haven't seen that show in quite some time but if I had landed that I think it would be like winning the lottery!

You could transform the slope into terraces and plant in those. For a slight slope you can create terraces with 4x4s. Pre-drill the 4x4s for rebars. It would help digging at least 2 ft deep post holes for the rebars, fill them with soft cement, and pound the rebars thru the drilled holes of the 4x4s into the cement. For a steep slope you would need 6x6s and anchors into the soil, Google for that.

Water off one of my neighbor's gutters washed his soil into my yard for years. My house is lower than his. After redoing my fence I bought pressure-treated 4X4s and rebar and fixed his slope as described above. Now none of his soil can bury my new aluminum fence.

Sounds like some sort of terracing is in order. A key principle is to have a system that lets water out before it builds up too much pressure but holds soil in (even with big poured concrete retaining walls they install "weep holes" and sometimes hidden drains so the water escapes and doesn't blow out the wall). Most interlocking stone systems allow water to seep out through the joints. Boulder walls have natural gaps to let water out (unless mortared). Wood terraces "bleed" water and often have hidden drains. Ways to terrace are endless and also depend on site conditions. I prefer stone to wood terracing since stone does not rot but it's more pricey and heavier. Steepness of the slope dictates to a large extent whether you can fix your problem with plants alone or if you need terraces. Soil conditions also. There are also various erosion control products you can try including compost filled, fabric tubes that are trenched and staked into place that slow the water down. Your budget, energy and skill level are also important. I don't think I've ever seen easy solutions to these problems but they can be fixed.